Dr. Syed Moin Hassan is a sleep medicine fellow at Brigham and Women’s Hospital. In this podcast on NPR, Dr. Hassan explains that each person has their own sleep cycle, or circadian rhythm. Like hair color, this cycle is determined by genetics. Light falling on the retina every morning helps synchronize the cycle, and helps the brain figure out when to instruct the pituitary gland to secrete melatonin, the hormone that puts you to sleep.
There is another element to sleep called the homeostatic sleep drive. In layman terms, this is an accumulation of waste products in the brain during waking hours, that eventually needs to be ‘cleaned up’ during sleep. Adenosine is one such waste product that drives sleep pressure.
For good quality sleep, these two cycles need to be in lockstep with each other. For some people, that means going to bed late, and not waking up early in the morning.