Short answer: your internal clock does not run at exactly twenty-four hours; morning light is what resets it each day, and it sets when sleepiness will arrive that evening. Aim for 5–10 minutes outdoors on a clear day, 15–30 under cloud, within an hour of waking. The mirror adjustment matters just as much: very little evening light is needed to shift the clock, and ceiling lights are the main offender.
What light actually sets
Two things get confused constantly. There is sleep pressure — the longer you've been awake, the more the drive to sleep accumulates — and there is the circadian clock, which decides when that drive is allowed to show up. You can be exhausted and completely unable to sleep: pressure high, clock saying no.
Light does not touch pressure. It sets the clock, and nothing else does so with the same authority. A disciplined bedtime applied to a shifted clock is just an hour spent lying in the dark, waiting.
The cleanest evidence: a week without electricity
In 2013 a team published in Current Biology an experiment of disarming simplicity: send adults camping for a week with no artificial light at all, and measure their clock before and after. The result is unambiguous — the clock re-entrains to sunrise and sunset within days, and the drift accumulated by indoor living disappears.
What that study demonstrates is not that you should go camping. It is that modern misalignment is not an individual failing: it is a light-exposure problem, and it is corrected by light exposure.
At the other end of the scale, an analysis of more than 400,000 UK Biobank participants, published in the Journal of Affective Disorders in 2021, finds the same direction in population data: time spent in daylight is associated with better sleep, an earlier chronotype and steadier mood. An association, not causal proof — but it points the same way as the experiment.
The dose, in practice
| Situation | Aim for | Note |
|---|---|---|
| Outdoors, clear sky | 5–10 min | Coffee on the doorstep does it. |
| Outdoors, overcast | 15–30 min | A grey day is still far brighter than a living room. |
| Through a window | Weak | Glass removes a substantial share of the intensity. |
| Light therapy lamp | Per device | A winter or night-shift substitute, not a first choice. |
| Sunglasses | Take them off | Unless there's a medical reason, or glare at the wheel. |
A second dose in early afternoon strengthens the anchor without being as decisive as the morning one. It is the realistic catch-up for days spent indoors.
The mirror mistake: evening light
Half the adjustment happens at the other end of the day, and it is the half people skip. Two studies pin it down.
The first, published in The Journal of Physiology in 2000, established the response curve of the human clock to nocturnal light. Its conclusion fits in a sentence: the clock is far more sensitive than anyone assumed, and ordinary indoor levels — not floodlights — are enough to move it.
The second, published in the Journal of Clinical Endocrinology & Metabolism in 2011, compared exposure to room light against dim light before bed: room light delays melatonin onset and shortens the window during which the body produces it. Not a screen, not a medical lamp: the living-room ceiling fitting.
The useful detail: the retinal cells involved respond mostly to light coming from above, which gives a simple, free instruction. Two hours before bed, overhead lights off, lamps at hip height, as dim as you can still work with. That is worth more than any software filter.
Wake time, not bedtime
A direct and counter-intuitive consequence: the variable you actually control is not bedtime, it is getting up. Nobody decides to fall asleep; you decide to get up, and you decide to go outside. Fixing a constant wake time — weekends included — gives the clock the one stable reference it needs, and it is also the first move in behavioural protocols for insomnia.
A two-hour lie-in on Saturday isn't neutral: it is the equivalent of imposing a transatlantic flight on your clock every weekend, with Monday to pay the bill.
Two caveats. Heavy daytime sleepiness despite adequate sleep, or loud snoring, is not a light problem: get it assessed. And with retinal disease, photosensitising medication or bipolar disorder, increasing bright-light exposure is a conversation to have with a doctor first.
How Sillon uses it
"Morning light, outdoors" is one of the library's grade A practices, and it opens nearly every sleep-related path: it is the best effort-to-effect move available, and it needs no equipment. Its twin card, "Dim the evening", arrives at the other end of the day. The app gives the dose, the time window, the reason and the references — not just the instruction.
Sources
- Wright KP Jr et al., "Entrainment of the human circadian clock to the natural light-dark cycle", Current Biology (2013)
- Burns AC et al., "Time spent in outdoor light is associated with mood, sleep, and circadian rhythm-related outcomes", Journal of Affective Disorders (2021)
- Zeitzer JM et al., "Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression", The Journal of Physiology (2000)
- Gooley JJ et al., "Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans", The Journal of Clinical Endocrinology & Metabolism (2011)
- Trauer JM et al., "Cognitive Behavioral Therapy for Chronic Insomnia: A Systematic Review and Meta-analysis", Annals of Internal Medicine (2015)
The morning move, prompted at the right hour
Sillon puts light in the right place in your day, with the dose and the reason. Free, offline, no account.