Sleep pressure / two-process model

The two-process model, proposed by Alexander Borbély in 1982, explains sleep and wake as two forces working together. Process S is homeostatic sleep pressure. It builds up while you are awake, as adenosine and other sleep signals pile up, and it drains away while you sleep. Process C is the circadian signal, made by your suprachiasmatic nucleus. It runs on a roughly 24-hour cycle and pushes alertness up toward evening, fighting the rising sleep pressure so you stay awake. You fall asleep when Process S grows past the circadian alerting threshold. The model neatly explains the post-lunch dip, the rebound deep sleep after a missed night, and the sharp wake-up boundary in the morning. It is still the leading framework for sleep and circadian research.

Last reviewed: June 22, 2026

This definition is educational and is not medical advice, a diagnosis, or treatment. Talk to a doctor about any health decisions. Read our full medical disclaimer