In a 2017 literature review aptly named Waking up is the hardest thing I do all day, “sleep inertia” was defined as “the transitional state between sleep and wake, marked by impaired performance, reduced vigilance, and a desire to return to sleep.”
Sleep inertia is a natural response that can last anywhere from minutes or hours, depending on the person. Sleep researchers aren’t totally sure what causes or intensifies sleep inertia, but they suspect it has something to do with the body’s circadian rhythm, also known as our internal clock.
In one small study on emergency workers who need to be “on” as soon as they wake up, the effect of sleep inertia was tested throughout the night. Twelve workers were woken up at various points of the night and early morning and asked to solve math equations (sounds fun, no?).
Their cognitive performance seemed to be lower when they were woken up during the “biological night,” when their bodies’ circadian clocks were telling them it was time to sleep, indicating that this clock can contribute to grogginess and increased processing time.
Daylight is one big factor that winds up our internal clocks, and we humans have adapted to being awake during the day and asleep after dark. As a result, our bodies produce different hormones during the day and evening that signal it’s time for rest. Two hormones that are integral to the sleep-wake cycle are melatonin and cortisol: When things are running smoothly, melatonin production ramps up at night to make us tired, and cortisol levels are highest in the morning and gradually taper as the day goes on.
This is why people who travel to new time zones or take on a night shift at work find it difficult to get a new sleep schedule going at first. Their internal clocks don’t match up to outer ones.