The timing, duration, and consolidation of human sleep result largely from the interaction of
2 sleep regulatory systems: the sleep-wake homeostat and the circadian timing system.
When these 2 processes are aligned and functioning optimally, they allow adults to achieve
a long, consolidated bout of wakefulness throughout the day and a long and consolidated
sleep episode at night. Changes to either process, or a change in how the 2 processes
interact, can result in an inability to fall asleep at the desired time, difficulty remaining asleep …