Sleep is often described as though a single mechanism switched it on and off. Two separate systems are involved, and they can act against each other, which explains most ordinary sleep difficulties.
One system tracks how long you have been awake
From the moment of waking, a pressure to sleep begins accumulating, rising steadily throughout the day and dissipating during sleep.
The accumulation is associated with the build-up of certain by-products of brain activity, which is why the process is so closely tied to time awake rather than to anything else.
This system alone would produce sleep that got progressively more likely until it happened, with no particular preference for night.
The other tracks time of day regardless of sleep
A separate internal clock runs on an approximately twenty-four hour cycle and generates alerting signals that vary by time of day.
That signal rises through the afternoon and evening, peaking not long before habitual bedtime, and then falls away sharply.
Its purpose is to oppose accumulating sleep pressure during the day, which is what allows sustained wakefulness rather than progressive decline.
Sleep happens where the two align
Falling asleep easily requires high accumulated pressure at a time when the clock's alerting signal is low, which normally occurs in the late evening.
When pressure is high but the clock is still signalling alertness, the result is exhaustion combined with an inability to sleep, which is a familiar and confusing state.
The reverse produces early waking, where the clock has begun its morning rise while pressure has not fully dissipated.
Common habits pull them apart
Napping late in the day discharges accumulated pressure without advancing the clock, which removes one half of the requirement at bedtime.
Irregular wake times move the clock while leaving pressure to accumulate on a different schedule, which is why weekend lie-ins are followed by difficult Sunday nights.
Light exposure at unusual hours shifts the clock directly, meaning evening light and dark mornings both work against the alignment.
Why fixed wake time is the standard advice
The wake time anchors both systems at once, setting when pressure starts accumulating and providing the light exposure that keeps the clock in position.
Fixing it is therefore more effective than fixing bedtime, since bedtime is an outcome of the two systems rather than an input to them.
Persistent difficulty despite a regular schedule, or excessive daytime sleepiness, is worth raising with a doctor, since several treatable conditions present that way.