Imaging homeostatic sleep pressure and circadian rhythm in the human brain

It is well-known that our sleep-wake patterns and alertness level during wakefulness are mainly modulated by two interactive processes, a homeostatic sleep debt process (S) refers to the drive for sleep that increases as a saturating exponential when we stay awake and decreases exponentially when we sleep, and a circadian processes (C) refers to the internal oscillatory rhythm that runs about 24 hours and can be reset by the environmental light (1-3).