TL;DR

Light is not just how you see the world. It is the single most powerful signal your biology uses to set the time of day, prime your hormones, regulate your mood, and protect your metabolism. The problem? Modern indoor environments are biologically equivalent to living in permanent twilight. This edition covers the neuroscience of light and circadian biology, and three high-ROI protocols you can start this week. This is an important one so I have included a bit of nerdy terminology including brain regions to give you the full background. If you're in a rush, skip down to the key takeaways near the end.

(And before we dive in, I was travelling in Australia last week, which is why your inbox was quieter than usual. Normal transmission resumes.)

Your Office Is A Cave

Here's a question. What do you think the light level is in your office right now?

A well-lit modern office delivers somewhere between 100 and 500 lux. Your brain's circadian biology evolved under direct outdoor daylight, which delivers between 10,000 and 100,000 lux depending on cloud cover and time of day. Even a grey, overcast New Zealand morning outside produces roughly 1,000-10,000 lux.

You are spending your most cognitively demanding hours in an environment that is roughly 100 times dimmer than the one your brain was designed to operate in.

That gap is not a minor rounding error. It has measurable consequences for your cortisol rhythm, your serotonin, your alertness, your mood, and by the time the sun goes down, your melatonin, your insulin sensitivity, and your HRV.

Light is not a passive feature of your environment. It is one of the most powerful environmental factors our biology responds to. And most of us are both chronically underdosed during the day and significantly overdosed at night. This edition is about fixing that.

Think of this way, one of the most consistent environmental variables that has been present during our entire evolution from microbes all the way through to humans has been the consistent rising and falling of the sun. It's no wonder why it has such a profound impact on our biology.

The Basics: How Your Brain Reads Light

Most people understand light through vision. You open your eyes, photons hit your retina, you see what's in front of you. But there is a second, completely separate light-sensing system in your eye that has nothing to do with seeing anything at all. It is entirely dedicated to telling your brain what time it is.

In 2002, a researcher named David Berson and colleagues discovered a specialised subset of retinal cells called intrinsically photosensitive retinal ganglion cells, or ipRGCs. These cells contain a photopigment called melanopsin, and they do not send signals to your visual cortex. They project directly to your hypothalamus, specifically a tiny bilateral structure called the suprachiasmatic nucleus, or SCN.

The SCN is your master biological clock. It contains approximately 20,000 neurons and it synchronises virtually every biological rhythm in your body from hormones to autonomic balance from immune function to metabolism, even gene expression. It does this by integrating light signals from your ipRGCs every single day.

Here is the critical detail. The human circadian period is not exactly 24 hours. It averages approximately 24.18 hours (Czeisler et al., 1999). Without a daily light signal to reset it, your clock drifts. You gradually shift later and later. That is why even a week of camping with no artificial light shifts people's sleep timing forward by roughly two hours and dramatically improves sleep quality (Wright et al., 2013, Current Biology, DOI: 10.1016/j.cub.2013.06.039). It's not that their internal clocks were broken, it's that they just finally got the correct signal.

The SCN receives its light input and then orchestrates your entire biological day through downstream projections to the pineal gland (melatonin), the HPA axis (cortisol), the autonomic nervous system, and peripheral clocks throughout your liver, pancreas, adipose tissue, and muscle. Every single one of these systems runs on a 24-hour schedule, and light is how that schedule gets set.

For the Geeks: The Pathways Worth Knowing

A couple of mechanistic details that I think are genuinely worth understanding, not just for interest but because they directly shape the protocols.

The phase response curve (PRC). Your circadian system does not respond uniformly to light. Light in the hours before your core body temperature minimum (typically around 4-5am for most people) shifts your clock later. Light after that minimum shifts your clock earlier. Khalsa et al. (2003) produced the definitive human PRC, showing phase delays of up to three hours and phase advances of up to two hours from a single bright light exposure. This is why morning light is the protocol. Not because morning is inherently special, but because it falls after your temperature minimum and drives your clock earlier which makes it easier to fall asleep that night and wake the next morning.

The perihabenular nucleus and mood. In 2018, Fernandez and colleagues published a landmark paper in Cell showing that light affects mood and learning through distinct retinal pathways. The mood effects operate through an SCN-independent pathway to a structure called the perihabenular nucleus. This means light's influence on how you feel is not just a downstream consequence of better sleep or improved circadian alignment. There is a direct, anatomically separate neural circuit for it. The data is primarily from animal models and human translation is still emerging, but the mechanistic picture is compelling and an important distinction.

Spectral sensitivity changes with duration. Here is something that gets routinely oversimplified in wellness media. The popular narrative is that blue light (around 480nm) drives all the circadian effects, so if you filter blue light you have solved the problem (hence the popularity of blue light filtering glasses). The reality is more nuanced. Gooley et al. (2010) demonstrated that at the beginning of a light exposure, cone photoreceptors (which respond across the visible spectrum) contribute substantially to melatonin suppression and circadian phase shifting, not just melanopsin. Melanopsin becomes increasingly dominant only after 60-90 minutes of sustained exposure. This was confirmed and extended in a 2022 PNAS paper by St Hilaire and colleagues (DOI: 10.1073/pnas.2205301119). What it means practically: blue light filtering glasses help, but they are not a free pass to sit under blazing overhead lights all evening. The total light environment matters.

The How and Why: What Light Actually Regulates

The downstream effects of morning light on your biology are more far-reaching than most people realise.

Cortisol. Morning light enhances the cortisol awakening response (that sharp spike in cortisol that occurs within 30-45 minutes of waking and primes your brain for the day ahead). As I covered in the cortisol edition, a robust CAR is associated with better cognitive performance and more stable energy through the day. Figueiro and Rea (2012) showed that even 40 lux of blue-enriched morning light significantly enhanced the CAR compared to dim light. This is a surprisingly meaningful effect at a remarkably low intensity. In addition to setting the clock, morning light calibrates your primary performance hormone. This is really important for healthy cortisol levels.

Serotonin. Lambert and colleagues (2002) published a remarkable study in The Lancet in which they directly measured serotonin turnover in the internal jugular vein of living humans. Serotonin turnover correlated directly with the duration of bright sunlight on the same day of measurement, not accumulated over weeks, not seasonal. Same day. The effect was rapid and direct. This is one reason why a proper walk outside on a bright day has such an immediate mood effect. The mechanism is real and powerful for mood.

Alertness. Cajochen and colleagues (2000) established the dose-response curve for light and alertness. Notably, half of the maximum alerting effect of 9,100 lux was achieved at around 100 lux (meaning the relationship is logarithmic). Even modest amounts of light have disproportionately large alerting effects. But going from indoor to outdoor levels still produces a massive additional stimulus that most people never capture.

The Problem: What Modern Work Does to Your Light Biology

Let me give you the picture of what a typical senior leader's light day actually looks like.

You wake up, and if you're anything like most of the people I work with, the first light you encounter is your phone screen, probably somewhere between 5,000 and 8,000 Kelvin but delivered at around 100-200 lux from a few centimetres away. You get ready under bathroom lighting, maybe 200-300 lux. You commute in a car, probably while it's dark, in tinted and UV filtered windows. You arrive at your office. You sit under 300-500 lux fluorescent or LED lighting for the next eight to ten hours. You might eat lunch at your desk. You leave in the evening. You get home, the lights go on, the TV goes on, and you sit under overhead lighting of 200-400 lux until you go to bed.

At no point in that day did your circadian system receive a signal anywhere close to what it needs and what it has evolved to require.

During the day, you chronically underdosed. This never gives your SCN the bright signal required to robustly entrain your clock and maintain circadian amplitude.

At night, you overdosed. This provides a sustained light signal that suppresses melatonin and delays your circadian phase.

The international expert consensus (Brown et al., 2022, PLOS Biology), led by some of the most eminent researchers in the field including Charles Czeisler and Steven Lockley from Harvard, recommends a minimum of 250 melanopic lux during the day and less than 10 melanopic lux in the three hours before sleep. The average office delivers around 100-150 melanopic lux (well less that the recommended). The average home in the evening delivers 100-400 melanopic lux (well more than the recommended).

Most people are violating both recommendations simultaneously, every single day.

And your biology keeps score.

Melatonin suppression. Gooley et al. (2011) showed that exposure to normal room light (less than 200 lux) in the hours before bed suppressed melatonin by approximately 50% and shortened melatonin duration by approximately 90 minutes, compared to dim light. This is not bright light. This is the light level of a standard living room. Your brain is interpreting it as daytime.

Metabolic consequences. Mason and colleagues published a striking study in PNAS in 2022 in which they exposed healthy adults to a single night of moderate light (100 lux) during sleep. Just one night. The results: increased heart rate, reduced HRV, and measurably impaired next-morning insulin sensitivity. The proposed mechanism is straightforward, light during sleep drives sympathetic nervous system activation, which reduces skeletal muscle glucose uptake and elevates plasma glucose. A 2015 study by Morris et al. showed that circadian misalignment for just three days reduced insulin sensitivity by 22% in healthy adults. The American Heart Association issued a formal scientific statement in 2023 acknowledging that circadian disruption adversely affects cardiometabolic health. This is mainstream medicine now.

Circadian amplitude. This is the concept I want you to hold onto as the unifying frame. A strong circadian signal is not just about sleep. It is about the contrast between your biological day and your biological night, high peaks and low baselines. Bright days and dark nights produce a high-amplitude, robust circadian signal. Dim days and bright evenings flatten that amplitude. A flat circadian signal means blunted cortisol rhythms, impaired metabolic switching, degraded sleep architecture, and compromised cognitive performance. The operating system runs at severely reduced capacity.

This is the same principle I described in the cortisol edition and in the sleep edition: it is not about a single data point. It is about dynamic range.

Three High-ROI Protocols

Protocol 1: The Morning Light Anchor

The Science

I think my clients feel like I am a broken record with this but morning bright light is the most potent circadian zeitgeber available to you. It phase-advances your clock, enhances the cortisol awakening response, drives acute serotonin synthesis, increases daytime alertness, and, through the melatonin phase advance mechanism, makes it easier to fall asleep that evening (pay attention those of you with problems falling/staying asleep). The downstream benefits compound across the entire 24-hour cycle.

The key mechanistic detail: morning light falls after your core body temperature minimum, which places you on the advancing side of the phase response curve. This is the biological sweet spot. A consistent morning light signal is the single most powerful thing you can do to synchronise your circadian biology.

Burgess et al. (2003) demonstrated that three consecutive days of morning bright light produced a melatonin phase advance of approximately 2.5 hours. You do not need weeks of effort to see measurable results.

The Protocol

1. Within 60 minutes of waking, get outside for a minimum of 15 minutes. Ideally, this is 20-30 minutes.
2. No sunglasses (they filter the photons your ipRGCs need). You are not staring at the sun, you simply need to be in the outdoor light environment.
3. If outdoor access is genuinely not possible, use a 10,000+ lux SAD lamp positioned at eye level, roughly 50-60cm from your face, for 20-30 minutes. Many people build this into their morning coffee ritual.
4. This is a non-negotiable anchor, even on overcast days. Cloud cover does not bring outdoor light below 1,000 lux. Your retina knows the difference between outside and inside.
5. The protocol pairs with the CAR optimisation from the cortisol edition: consistent wake time, morning light, then delay your coffee by 90 minutes to allow the CAR to peak naturally.

Expected Outcome

Within 3-5 days, most people notice improved morning sharpness and more consistent energy through the day. Within 2-3 weeks, earlier sleep onset and more consistent wake times become noticeable. Track your resting HRV on waking, a rising trend over 2-3 weeks is a reliable signal that your circadian system is strengthening.

Protocol 2: The Evening Light Wind-Down

The Science

The melatonin suppression threshold is far lower than most people assume. Gooley et al. (2011) established that normal room light under 200 lux suppresses melatonin by 50% and compresses melatonin duration by 90 minutes. You do not need to be staring at screens to have a biologically significant evening light problem. The ceiling lights in your living room are doing the damage.

The expert consensus recommendation from Brown et al. (2022) is less than 10 melanopic lux in the three hours before habitual sleep. That is a dimly lit room with warm-spectrum bulbs, not a switch to darkness. It is achievable.

A secondary consideration is spectrum. While it is true (as covered above) that all visible light contributes to melatonin suppression, not just blue wavelengths, reducing the blue content of your evening light is still a meaningful lever. Switching to warm-spectrum bulbs (2,700 Kelvin or below) and positioning lights below eye level substantially reduces the melanopic stimulus without requiring darkness.

The Protocol

1. At three hours before your target sleep time, dim all overhead lights in your home. This is your trigger point, set an alarm/calendar event if needed.
2. Switch to warm-spectrum bulbs at 2,700K or below. Lamps positioned low in the room (below eye level) are far less disruptive than ceiling lights.
3. If you are using screens after this point, enable night mode (this shifts the display toward warmer tones) and reduce screen brightness significantly. Blue-light-blocking glasses on top of this provide additional protection, though note that the evidence for glasses alone is mixed, it is the combination of reduced overall intensity and reduced blue spectrum that makes the difference. But aim for no screens for maximum response.
4. Aim for your bedroom to be as close to complete darkness as possible during sleep. Blackout curtains if streetlights are an issue. Cover any charging LEDs. Phone out of the bedroom.
5. One hour before sleep, your environment should feel noticeably darker and warmer than your daytime setting. That contrast is the signal.

Expected Outcome

Most people notice a meaningful shift in subjective sleepiness at an earlier time within the first week, this is your melatonin onset re-advancing. Sleep onset latency (the time between getting into bed and falling asleep) typically reduces. Watch for nighttime heart rate and HRV. The Mason et al. (2022) data suggests that even modest reductions in sleep-time light can shift the sympathovagal balance back toward parasympathetic, which shows up as improved overnight HRV.

Protocol 3: The Daytime Light Maximisation Block

The Science

The daytime side of the light equation is at least as important as the evening side, and far more neglected. This is the dynamic range principle in practice: bright days strengthen the circadian signal and make the brain more resistant to the disruptive effects of evening light. Chang et al. (2011) demonstrated that individuals adapted to bright daytime light showed less melatonin suppression from the same evening light stimulus compared to those who spent the day in dim indoor light. Your daytime light environment literally calibrates how vulnerable you are to evening light disruption.

The workplace alertness data is compelling too. Viola and colleagues (2008) compared standard office lighting with blue-enriched white light at 17,000 Kelvin across a workday. The blue-enriched group showed a 28.2% improvement in alertness, a 26.9% reduction in fatigue, and a 19.4% improvement in self-reported work performance. These are not trivial effect sizes for a single lighting change.

A larger analysis of the UK Biobank dataset (Burns et al., 2021), more than 400,000 participants, found dose-dependent associations between outdoor light exposure and improved mood, better sleep, and reduced circadian disruption.

The Protocol

1. Build at least two outdoor light breaks into your working day. These do not need to be long, 10-15 minutes each. A morning walk to get coffee, lunch outside, or a walking meeting are all valid. You are targeting a total of 20-30 minutes of genuine outdoor light exposure across the day, in addition to your morning anchor.
2. Position your primary work desk as close to a window as practically possible. Daylight through glass is filtered (UV is blocked, and the lux level falls significantly with distance), but it still delivers a meaningfully higher circadian stimulus than artificial overhead lighting.
3. During your deep work blocks, consider a daylight-spectrum desk lamp (5,000K or above) in addition to whatever your office provides. You are trying to push past the 250 melanopic lux threshold the consensus recommends. Most offices do not meet this minimum without supplementation.
4. Protect at least one lunchtime walk per week for a proper outdoor light dose, 20-30 minutes of uncovered outdoor exposure in the middle of the day provides a strong amplitude signal to your SCN, even if it doesn't have the phase-shifting effect of morning light.
5. Think of this as your daytime charging session. Your circadian biology runs on light contrast. Every outdoor minute you accumulate during the day is building the amplitude of a signal that will help you fall asleep faster that night and wake sharper the next morning.

Expected Outcome

The alerting effects are noticeable within days. Most people find that consistent outdoor breaks significantly reduce the mid-afternoon energy slump (which is partly a normal circadian dip, but is substantially worsened by chronic indoor under-lighting and skipping morning light). The circadian amplitude effects build over 2-3 weeks. Track your afternoon alertness ratings (subjective 1-10 at 2pm and 4pm) as your benchmark, these should trend upward with consistent implementation.

Top Takeaways (In a Hurry? Start Here)

• Your circadian clock runs on a 24.18-hour period and needs a daily light reset. Without it, your biology drifts.
• Typical office lighting (100-500 lux) is 20-100x dimmer than what your circadian system evolved under. You are running your operating system in power-save mode.
• Morning bright light is the most powerful single intervention for cortisol, serotonin, alertness, and sleep onset. Get outside within 60 minutes of waking.
• Normal room light in the evening suppresses melatonin by 50% and compresses its duration by 90 minutes. Dim and warm your home environment three hours before bed.
• A single night of 100 lux during sleep impairs next-morning insulin sensitivity and reduces HRV. Your bedroom darkness is a metabolic variable.
• Daytime and evening light are one integrated system. Bright days make you more resilient to evening light. Both sides matter.
• Blue light filtering helps, but it is not the whole story. All visible light suppresses melatonin at sufficient intensity. Reduce overall evening light level, not just blue wavelengths.

Objective Markers to Track

If you want to measure your progress rather than guess:

• Resting HRV on waking: trending upward indicates improved autonomic balance and circadian robustness
• Sleep onset latency: time to fall asleep should reduce within 1-2 weeks of the evening protocol
• Resting heart rate overnight: should reduce with better sleep-environment darkness
• Subjective alertness at 10am and 2pm: rate 1-10 daily, morning light should shift both scores upward within a week
• Wake ease: subjective difficulty of waking (1-10) should improve as circadian phase advances

If you want help building these into a personalised performance blueprint alongside the rest of your biological operating system, that is exactly what we do at Second Summit. Feel free to reach out, I would love to hear how you get on with the protocols.

Until next week.

Simon

Further reading: Brown et al. (2022) is the definitive expert consensus on indoor light exposure and is open access, worth bookmarking. For the sleep side of this picture, see the Sleep Advantage edition. For how morning light interacts with your cortisol rhythm, the Cortisol edition has the full picture.