How a night is actually built, and why the first half matters most
Most advice about sleep treats the night as one long block to be protected. It is not. A night is a sequence of cycles, and the two halves of it do different jobs — which is why losing the beginning of a night is not the same as losing the end.
The cycle, and how many of them
The INSV, the French national institute for sleep and vigilance, describes a first cycle of about ninety minutes, with a night made up of three to five cycles in succession. Inserm gives a slightly wider range: three to six cycles of sixty to one hundred and twenty minutes. The spread matters more than the average — a person whose cycle runs closer to sixty minutes and one whose cycle runs closer to two hours do not need the same bedtime to wake at the same hour.
What each stage takes up
Across a whole night, Inserm gives this distribution: stage N1, the transition into sleep, four to five per cent; stage N2, light sleep, forty-five to fifty-five per cent; stage N3, deep sleep, sixteen to twenty per cent; and REM sleep, twenty to twenty-five per cent. Light sleep is therefore the bulk of the night, which surprises people who assume deep sleep should dominate.
Deep sleep is associated with physical recovery. REM sleep is, according to Inserm, the stage conducive to dreams — in particular those still remembered on waking.
The two halves are not interchangeable
This is the detail that explains a great deal. The INSV notes that deep sleep is very abundant at the beginning of the night and disappears completely in the early morning, whereas REM sleep is brief at first and takes up a growing share as the night goes on.
The practical consequence is blunt. Going to bed two hours late and waking at the usual time does not remove two hours evenly from the night: it removes them from the part that carries physical recovery. Sleeping in at the weekend, conversely, mostly adds REM sleep. The two are not substitutes, and no amount of lie-in repays a series of late nights in kind.
Waking between cycles is normal
Brief passages through wakefulness between two cycles are a normal part of the architecture, not a symptom. Most are never remembered. The ones that are remembered tend to be those where something — a noise, a light, a full bladder, a phone — turned a two-second surfacing into a ten-minute one. That is the useful lever: not preventing the awakening, but keeping it short.
The clock underneath
None of this runs on a twenty-four-hour schedule by itself. According to Inserm, the cycle the body imposes spontaneously lasts between twenty-three hours thirty and twenty-four hours thirty depending on the individual, with an average estimated at twenty-four hours ten minutes. Left to itself, that internal clock drifts. What resets it every day is light.
Which is why the single most effective change is also the least expensive: daylight in the morning, and dimmer light in the evening. Inserm underlines how sensitive the system is — it was long believed that more than a thousand lux were needed to affect it, but a few lux are enough. The brightness of a full-moon night can block the secretion of melatonin.
What to do with this
Three things follow, and none of them require buying anything. Protect the first half of the night before the second. Treat a brief awakening as normal and give it nothing to feed on. And get light early, because the clock that decides when you will be sleepy tonight is set in the morning.
Sources: INSV (Institut National du Sommeil et de la Vigilance); Inserm, sleep dossier.