Your sleep cycle is primarily regulated by circadian clocks that drive rhythmic fluctuations in hormone levels. Humans have several of these, but we only really understand the main one, which is in your hypothalamus, way down near the base of your brain.
Here’s how it works. Your eyes contain special photosensitive cells (in addition to the rods and cones you use for normal vision) which reach back into your hypothalamus. When light touches your eyes, these cells alert the hypothalamus, which uses the information to decide whether it’s daytime based on a roughly 24-hour cycle. When your hypothalamus decides it’s night time, it tells your pineal gland to secrete melatonin. And melatonin makes you sleepy.
Artificial lighting at night confuses your hypothalamus. It thinks, oops, I’m out by a couple of hours. So it tries to correct by making you sleep longer in the morning. People also often shut out morning light with curtains, confusing their hypothalami even more. That night, your hypothalamus sticks to its new schedule, so you get sleepy later, so you keep the lights on later, and your hypothalamus decides to shift your cycle even further.
In experiments where people are kept away from natural lighting, they tend to maintain a circadian rhythm about 20 minutes too long. So it seems our natural clock isn’t that reliable to begin with; we’re heavily dependent on natural lighting to keep it accurate.
By the way, circadian clocks also decrease your heart rate, lower your core body temperature and lower your cortisol (stress hormone) levels each night. In fact, recent research shows that screwing up your circadian clock disrupts the function of over 1500 genes.
It’s been suggested that modern sleep patterns are highly unnatural, resulting from the prevalence of artificial lighting and chronic sleep deprivation. It’s claimed that pre-industrial and tribal societies…