Many people experience a groggy, foggy, out-of-sorts feeling after oversleeping—that is, getting more sleep than they normally do. The research is scantier on the actual physiological impact of oversleeping than on that of insufficient sleep, but there are a few possible explanations for the phenomenon and its ramifications.
Sleep Drunkenness or Inertia
The fuzzy-headed, droopy-lidded state following oversleeping—grogginess—is sometimes called “sleep drunkenness.” This occurs when our systems seem strung between full-on sleep and alert wakefulness, or “sleep inertia,” often specifically referring to the mental impairment one can experience after abruptly waking from deep sleep.
Sleep inertia often seems to be connected to the phase of the sleep cycle you’re experiencing just prior to being awoken. Specifically, those who awake from slow-wave, or delta, sleep—technically stages 3 and 4 of the typical sleep cycle—are often groggy for some time afterward. Slow-wave sleep comes on the heels of stages 1 and 2, the light sleep that transitions us from wakefulness to slumber, and can be described as “deep sleep”; it’s often most extended in the early hours of our sleeping night. Stages 1 through 4 are the non-REM (rapid eye movement) portion of our sleep cycle. REM sleep differs strikingly from slow-wave sleep in that our cerebral cylinders are highly active. This is the stage of full-on dreaming. And during this stage some of our muscles are actually partly paralyzed (presumably to prevent the body from physically “performing” our dreams, a situation with obvious potential for danger).
If your oversleeping has taken you into an extended repetition of these cycles, therefore, and you happen to wake during slow-wave sleep, you may well feel groggier and drowsier than had you awoken during light-sleep or REM phases.
In some cases, grogginess following oversleeping may be actually be a symptom of a broader sleep deficit. Many Americans, from high-school students to working professionals, seem to follow a pattern of insufficient sleep during the school or work week and extended, “catch-up” hibernation during the weekend. This so-called “weekend oversleep” often indicates the person is not getting enough shut-eye during the week, and as a result may not be functioning as efficiently as they believe.
The Power Nap
Everyone’s particular sleep patterns are different, but many people notice that a short nap—say, of 30 minutes or less—leaves them feeling more alert and awake afterward than a nap of longer duration. The brief, refresher bout of light sleep—the classic cat nap or power nap—can be a healthful part of the day. However, it’s certainly easy, particularly if you’ve not been getting enough nighttime shut-eye, to oversleep and enter slow-wave territory (thus making a bout of sleep-inertia grogginess all the more likely upon waking).
Chronic sleepiness during the day or chronic oversleeping is sometimes called hypersomnia, and can be a real problem for sufferers. They may feel compelled to nap regularly, even in the middle of a work shift, and yet glean little refreshment from the rest. Hypersomnia is a condition that might be caused by any number of underlying issues, such as sleep apnea, narcolepsy, or depression.
A host of sleeping disorders and special conditions can promote grogginess. For example, hypothyroidism, in which the body produces an insufficient level of thyroid hormones, commonly promotes weariness and fatigue. Your body’s natural circadian rhythm, which describes normal fluctuations of temperature, hormone activity, and other physiological processes, can also play a significant role in oversleeping and tiredness. Delayed sleep-phase disorder occurs when an individual’s circadian rhythm doesn’t jive with the societal schedules they’re beholden to. A child’s particular innate rhythm may have them experiencing a circadian “trough”—like reduced body temperature—during the morning interval when they must awake and get ready for school. This can make them more likely to “oversleep”—at least in the context of the standard schedule—and also to feel tired and groggy during the day. People with delayed sleep-phase disorder often find success in adjusting their schedules to synchronize with their particular circadian rhythm. They may end up taking night jobs or otherwise working around the “9-to-5” paradigm.
The exact impacts that oversleeping has on our health and lifestyle are not entirely clear, and the issue is complicated by strong individual variations in metabolism and circadian rhythm. Some people seem to be able to get away with sleeping five hours on weeknights and crashing half the day on weekends, while others may be more detrimentally impacted by such a sleep deficit. All in all, oversleeping every once in awhile is unlikely to significantly affect our day-to-day functioning, and the average person may well benefit from submitting to the body’s occasional request for another hour of shut-eye.