Scientists are still sorting out exactly why animals need sleep, but it’s readily apparent that we do. In the 1980s, a famous study deprived rats of sleep for 30 days — and every single one of them died. In people, short-term sleeplessness can cause blood pressure to increase and body temperature to drop; it weakens the immune response and impairs cognitive function.
Just about everything in the human body is set up to help us rest at night, starting with the eyes. A study in the early 2000s found that our eyes have specialized cells — called “intrinsically photosensitive retinal ganglion cells” — that alert our brain to the amount and quality of light in which we’re immersed. Interestingly, they work even in people who are visually impaired. That’s why blind people are able to stick to a day-night rhythm, even though they can’t actually see light. By contrast, people who have lost their eyes entirely, to a tumor or accident, will find their sleep cycles hopelessly adrift.
When they sense light, the specialized retinal ganglion cells send a signal to a section of the brain that serves as the body’s master clock, the suprachiasmatic nucleus. The SCN is the conductor of the daily symphony that is our circadian rhythm. Though we associate this cycle with sleep (or, for some shift workers, the lack thereof), it plays a role in far more: in body temperature, blood pressure, hormone release, cognitive ability. Each fluctuation is carefully scheduled to happen when we need it. Even our cells are keeping time; appropriately named CLOCK genes produce specific proteins in a 24-hour cycle.It all comes to a crescendo around 11 p.m., when the sleep drive that has slowly been accumulating throughout the day reaches a sort of agreement with the various chemical signals coming from our circadian system: It’s time to go to bed.
“The circadian rhythm is about every aspect of our physiology and behavior,” says Russell Foster, the chair of circadian neuroscience at the University of Oxford. “And light is critically important in locking that internal rhythm to the external world.”
If you start messing around with light, though, the entire symphony is thrown wildly off tempo. A shift worker who steps out of work at 6:30 a.m. to see the sun rising overhead will start to feel their blood pressure rise and focus sharpen, even as their sleep drive is saying to wind down. Torn between the two impulses, sleep will become infuriatingly elusive.