Added October 2020
Our bodies follow an internal biological clock called the circadian rhythm: body temperature and wakefulness peaks during the day, dips for a time in early afternoon, and begins to drop again in the evening. This rhythm can be altered when light striking the retina signals the suprachiasmatic nucleus to suppress the pineal gland’s production of the sleep hormone melatonin.
Wakefulness is also affected by an independent system known as sleep pressure, which responds to how long you’ve been awake. While you’re awake, the neurons in your brain fire away and produce adenosine as a byproduct. Adenosine is monitored by your nervous system through receptors. Typically, when adenosine levels drop, your body will signal you to start to prepare for sleep. Caffeine works by binding to your adenosine receptors, tricking your body into thinking that your adenosine levels are still high, and so that it’s not yet time for sleep.
There are four distinct sleep stages, which we cycle through every 90 minutes: NREM-1 (brief; fleeting images); NREM-2 (20min); NREM-3 (30min, minimal awareness); and REM (10min, story-like dreams).
We almost always have dreams during REM sleep, though we typically forget them by morning. (We also typically forget anytyhing that happens during the five minutes just before we fall asleep.) Dreams usually involve negative events or emotions, such as repeatedly failing in an attempt to do something; being attacked, pursued, or rejected; or experiencing misfortune.
Sleep helps repair brain tissue, restore the immune system, and consolidate memories. Sleep loss also affects our energy, hunger, mood, attention, and reaction time.
Resources: Myers and Dewall 2015; Sleep Foundation; melatonin