Двопроцесна модель сну: Процес S зустрічається з Процесом C

Two clocks, not one

You feel sleepy after roughly 16 hours awake regardless of what the clock on the wall says — but you also feel far more alert at 9 PM than at 3 PM, despite having been awake for the same total duration in each case. Neither fact alone explains human sleep timing; together, they point to two separable biological processes running in parallel. In 1982, Alexander Borbély proposed the two-process model: a homeostatic sleep drive that only cares how long you've been awake, and a circadian oscillator that only cares what time of day it is. Sleep happens where the two collide.

Process S: adenosine on a stopwatch

Process S is the homeostatic sleep pressure — it rises while you’re awake and falls while you’re asleep, tracking a single underlying chemical signal: adenosine accumulating in the basal forebrain as neurons burn ATP. Caffeine feels like it "wakes you up," but it doesn’t remove any adenosine — it just blocks the A1 and A2A receptors that would otherwise sense it, which is why a delayed crash arrives once the caffeine’s roughly five-hour half-life clears and all that backlogged adenosine binds at once.

// Waking: S rises toward S_max with time constant tau_w ~ 18.2 h S(t) = S_max - (S_max - S0) * exp(-t / tau_w)

// Sleeping: S falls toward S_min with time constant tau_s ~ 4.2 h S(t) = S_min + (S0 - S_min) * exp(-t / tau_s)

// A normal 16h-awake / 8h-asleep day: // S climbs ~0.20 -> ~0.68 while awake // S falls ~0.68 -> ~0.18 while asleep Sleep recovers faster than sleepiness accumulates — the sleeping time constant is roughly four times shorter than the waking one — which is why a single good night’s sleep can undo a day of fatigue almost entirely. The EEG signature of Process S is slow-wave activity (0.5–4.5 Hz delta power) during NREM sleep: it’s highest in the first sleep cycle, when S has just peaked, and decays through the night in near-perfect lockstep with the model’s prediction.

// Waking: S rises toward S_max with time constant tau_w ~ 18.2 h
S(t) = S_max - (S_max - S0) * exp(-t / tau_w)

// Sleeping: S falls toward S_min with time constant tau_s ~ 4.2 h
S(t) = S_min + (S0 - S_min) * exp(-t / tau_s)

// A normal 16h-awake / 8h-asleep day:
//   S climbs   ~0.20 -> ~0.68  while awake
//   S falls    ~0.68 -> ~0.18  while asleep

Process C: the clock that doesn’t care how tired you are

Process C is generated by roughly 20,000 neurons in the suprachiasmatic nucleus (SCN) of the hypothalamus — a self-sustaining oscillator that keeps running on its own near-24-hour rhythm even in total darkness, entrained day to day by light hitting specialised retinal photoreceptors. Unlike Process S, it is completely indifferent to your sleep history; it approximates a simple cosine, peaking for alertness in the afternoon and bottoming out around 4–6 AM.

C(t) = C_mid + A * cos(2*PI * (t - phase) / T) // T ~ 24.2 h (free-running period in constant darkness) // phase peak alertness ~midday, nadir ~4-6 AM Critically, Process C doesn’t just sit passively in the background — in the early evening it actively counteracts the day’s accumulated Process S, holding off sleep onset until after dark in what chronobiologists call the "wake maintenance zone." That’s why you can feel wide awake at 9 PM despite having been up since 7 AM, and then feel a wall of sleepiness hit at 11 PM once the circadian push fades. Individual variation in the phase of this oscillator produces chronotypes — morning larks are phase-advanced, night owls phase-delayed, and the distribution skews later in teenagers due to a genuine adolescent shift in SCN timing, which is a large part of why early school start times hurt learning.

C(t) = C_mid + A * cos(2*PI * (t - phase) / T)
//   T     ~ 24.2 h   (free-running period in constant darkness)
//   phase       peak alertness ~midday, nadir ~4-6 AM

Де точка перетина двох ліній: зворотний вимикач

Початок та кінець сну не регулюються єдиним фіксованим порогом — самі пороги коливаються разом із циркадним ритмом, відкриваючи нічну "слідову браму" і закриваючи її знову біля світанку. Неврологічно, перехід відбувається різко, а не поступово, оскільки він реалізований як зворотний вимикач: ядро ​​вентролатерального преоптичного (VLPO) пригнічує мозкові центри, що сприяють пробудці, і ці центри, що сприяють пробудці, взаємно пригнічують VLPO назад. Кожен бік придушує інший, тому система перемикається між двома стабільними станами — повністю неспала або повністю спала, а не залишається на невизначеності надовго, як світловий вимикач, а не диммер.

Оксетинні нейрони в латеральному гіпоталамусі діють як стабілізатор цього перемикача, підсилюючи будь-який поточний активний стан; їх втрата є основною причиною нарколопії, де зворотний вимикач спрацьовує безладно і пацієнти перемикаються між пробудженням та сном REM з малою попередженням. Дослідження пошкоджень підтверджують двівалідну архітектуру безпосередньо: знищення SCN у тварин не зупиняє сон, але воно пригнічує щоденний ритм — залишається лише короткі, фрагментовані сновидіння, які регулюються Процесом S само по собі, без більш узгодженого дня/ночі.

Deprivation, debt, and jet lag

During sustained wakefulness, Process S climbs monotonically and cognitive performance falls in step — after roughly 24 hours awake, S approaches its ceiling and psychomotor vigilance impairment is comparable to a 0.1% blood alcohol level.

Crucially, during chronic partial sleep restriction (six hours a night for two weeks), objective performance keeps declining while subjective sleepiness plateaus — people stop feeling as impaired as they actually are, a genuinely dangerous mismatch that the two-process model predicted before it was confirmed experimentally.

Jet lag is a Process C problem, not a Process S problem — your sleep pressure adapts to a new time zone within a day or two, but the SCN's phase re-entrains far more slowly, at roughly 1 hour per day flying east and 1.5 hours per day flying west.

The asymmetry follows directly from the free-running period being slightly longer than 24 hours: westward travel asks the clock to delay, which is the direction it already wants to drift, while eastward travel demands a phase advance against that natural tendency.

Shift workers face the cruelest version of this mismatch — because their light exposure never consistently supports one phase, Process C often never fully re-entrains at all, a state of chronic circadian misalignment linked to elevated cardiovascular and metabolic disease risk.

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