Sleep, a user’s manual: what neuroscience has actually established

There is a simple question to ask any seller of a “sleep” supplement: which of the three systems does your product act on? Most cannot answer, because most products act on none of them. Here are the three systems, what we know about each, and what actually sets them.

Three systems, not one

Sleep is not a switch. It is the result of three partly independent mechanisms, and that is precisely why “I sleep badly” does not describe one problem but at least three different ones.

1. Sleep pressure (the homeostatic process)

From the moment you wake, your brain accumulates adenosine, a by-product of its own functioning. The more it builds up, the stronger the urge to sleep. Sleep clears it: you wake “emptied” of the need to sleep. It is a pressure, mechanical, that rises with the hours you are awake.

Caffeine does not remove adenosine: it occupies its receptors. It masks a signal that keeps rising — hence the collapse when it clears. It is a debt, not a gain.

2. The circadian clock (process C)

A nucleus of about twenty thousand neurons in the hypothalamus, the suprachiasmatic nucleus, imposes a roughly twenty-four hour rhythm on the whole body: core temperature, cortisol, melatonin, alertness, digestion. This clock does not set itself by the time on your phone. It sets itself by light, captured by dedicated retinal cells, distinct from those used for vision.

That is why a blind person can perfectly well see their rhythms drift out of sync, and why a week camping without artificial light realigns the clock more effectively than any supplement.

3. The arousal system

A network that maintains alertness when something demands it: noise, heat, pain, danger — or rumination. You can have enormous sleep pressure and a perfectly aligned clock: if this system is active, you do not sleep. It is the central mechanism of stress-related insomnia, and it is also why you fall asleep in front of a film and not at all once in bed, mind racing.

What happens while you sleep

A night is not a uniform block: it alternates cycles of about ninety minutes between deep slow-wave sleep and REM sleep. The public-facing work of the neuroscientist Matthew Walker made an important asymmetry widely known: deep slow-wave sleep dominates the first half of the night, REM sleep the second.

A very concrete consequence: going to bed two hours later while sleeping the same number of hours does not produce the same night. You preferentially cut deep sleep — the kind associated with growth hormone secretion, memory consolidation and physical recovery. And getting up two hours earlier mainly cuts REM sleep, involved in emotional regulation and procedural learning.

It is also during sleep that activity in the glymphatic system — the drainage of the brain’s metabolic waste — rises sharply. Maiken Nedergaard’s work on this system opened a whole field of research on the link between sleep and long-term brain health; the mechanisms are still debated, but the direction is clear.

Caffeine: the window almost nobody respects

Caffeine’s half-life is around five to six hours in the average adult, with enormous individual variation linked notably to the CYP1A2 gene. Translation: a double espresso at 4pm is still the equivalent of half an espresso in the blood at midnight.

A frequently cited study measured the effect of a 400 mg dose of caffeine taken zero, three or six hours before bed: even six hours before, total sleep time was significantly reduced — and participants did not always perceive that degradation. That is the most important point: how it feels underestimates the objective effect.

The practical rule we apply: last caffeine eight to ten hours before the target bedtime. For an 11pm bedtime, that puts the limit between 1pm and 3pm. And tea, cola drinks, dark chocolate and pre-workouts count in the total.

Temperature: the forgotten signal

Falling asleep comes with a drop in core temperature of about half a degree to a degree. The body achieves this by shedding heat through the extremities — hence warm hands and feet as you drop off. A bedroom that is too warm blocks that mechanism.

• 18 to 19 °C is the most commonly recommended range for the bedroom.
• A hot shower 60 to 90 minutes before bed speeds up falling asleep: the vasodilation it causes then lowers core temperature.
• Socks can paradoxically help: they make it easier to shed peripheral heat.

Light: the most powerful lever, and the most free

Two exposures count, in opposite directions.

In the morning: bright light within the hour after waking advances the clock and consolidates the rhythm. Outside, even on an overcast day, you easily exceed 10,000 lux; indoors you often cap below 500. The gap is more than an order of magnitude, and it explains a good deal of difficult winter mornings.

In the evening: light exposure delays the clock and delays melatonin secretion. The thresholds measured in the laboratory are low — the order of magnitude of domestic lighting is enough to produce an effect. The priority is therefore not to buy filtering glasses: it is to lower the intensity after dinner, and to get complete darkness in the bedroom.

Supplements: what the evidence allows us to say

Ranked by decreasing strength of data, for the intended use:

• Melatonin (0.5 to 1 mg). Solid evidence on phase shift and time to fall asleep, at a low dose. It is not a sedative: it is a clock signal. The 5 and 10 mg doses sold elsewhere improve nothing and increase residual grogginess. Real contraindications: see our comparison.
• Magnesium bisglycinate. Emerging evidence on sleep quality, solid evidence on correcting an insufficient intake. Useful if you get cramps or evening muscular tension; pointless if your intake is already good.
• Glycine (3 g). Emerging evidence, small numbers, a plausible mechanism (a fall in core temperature). Negligible cost, good tolerability: worth trying.
• L-theanine (200 mg). Acts on situational anxiety, and therefore on the third system — arousal. Relevant when it is rumination blocking you, not a lack of sleep pressure.
• Valerian, passionflower, “night” blends. Inconsistent data and doses generally far below those used in trials. We recommend nothing in this category.

When it is no longer about optimisation

If the difficulties have lasted more than three months, at least three nights a week, with an impact on your day, that is chronic insomnia. And there, the first-line treatment recommended by European and American learned societies is not a medicine or a supplement: it is cognitive behavioural therapy for insomnia (CBT-I).

It rests on counter-intuitive levers — restricting time in bed, stimulus control, working on beliefs about sleep — and its effects persist after the course ends, which is not the case for sleeping pills. Validated digital programmes exist. It is by far the most effective intervention on this page.

The protocol, in order of impact

1. Fixed wake-up time, seven days a week. It is the anchor of the clock. Free, and most often sabotaged by weekend lie-ins.
2. Bright light on waking, ten minutes outside. In winter, a 10,000 lux lamp takes over.
3. Complete darkness and silence at night. Mask and earplugs if the bedroom does not allow it.
4. Last caffeine eight to ten hours before bed.
5. Bedroom at 18–19 °C. A hot shower an hour before if needed.
6. Low light after dinner, and a finishing time for screens.
7. Alcohol: not within three hours of bed, and as little as possible.
8. One supplement, one only, for two weeks — and only if one specific point resists the seven rules above.

The first seven lines are free. That is deliberate: they carry most of the effect. An honest affiliate site should be able to write that down.