There’s a reason that your eyelids start to feel heavy in the evening, and pop open in the morning—sometimes even before you’ve gotten enough sleep. Blame it on your sleep/wake cycle (also known as your circadian rhythm).
The sleep/wake cycle is a daily pattern that determines when it’s time to sleep and when it’s time to be awake. For most humans, the ideal cycle includes seven to nine hours of sleep (typically at night) followed by 15 to 17 hours of wakefulness.
The body has a series of processes that set the stage for this cycle. For example, different chemicals and hormones rise and fall over the course of roughly 24 hours, causing you to feel tired (or awake) at predictable times. One such chemical is adenosine, which accumulates in your blood throughout the day. (Caffeine blocks the adenosine receptor, which is why coffee can help you fight the urge to snooze, even when you’re tired.) The longer you’re awake, the higher the level of adenosine in your blood, until eventually you tip into desperate-for-sleep mode. When you do drift off, your adenosine level drops, beginning the cycle again.
Melatonin is another hormone that helps to regulate the sleep/wake cycle. Your melatonin level starts to rise in the late afternoon and continues to creep up through the night, and then falls in the early hours of the morning, as your body prepares to wake. Turning on too many bright lights or screens in the evening hours may disrupt melatonin production, making it harder to sleep. That’s why a dark bedroom is ideal for sleeping, and bright lights in the morning (and stepping outdoors into the sunshine for a few minutes after you rise) can help you to wake up.
For people who don’t sleep at traditional times because of travel or shift work, some simple strategies can help you to adjust your sleep/wake cycle earlier or later, as needed. Turning on a bright light as soon as your alarm goes off helps to shift your cycle earlier, for example. For overnight workers who sleep during the day, wearing sunglasses home from work and then heading to bed with blackout curtains that block the sunlight can help to trick your body into preparing for sleep, even as much of the world is waking up.