sleep inertia
n. The grogginess and disorientation that a person feels for a few minutes after a sleep or long nap.

Example Citations:
‘Most people should only nap for 30 minutes or so, ‘ Camille says. ‘More than that and you’re getting into deeper sleep.’ And the problem with deeper sleep is waking up from it without succumbing to what’s known as ‘sleep inertia’ — that groggy, stumbling-around feeling.
—James B. Meadow, “Why Z’s? Because Workplace Napping Increases Productivity, Authors Say,” Denver Rocky Mountain News, November 10, 1999

There are two reasons naps can leave you feeling sleepy.

One, you might have a “sleep debt,” meaning you aren’t getting enough sleep in general. You’ll experience sleep debts if you mix late nights with early mornings.

The second reason has to do with something called “sleep inertia.” This is linked to how long your naps are.
—“Why am I more tired when I wake from a nap than when I go to sleep?,” The Toronto Star, November 22, 1998

Earliest Citation:
“Naps that contains deep or slow-wave sleep may be followed by a period of what we call sleep inertia. which is a kind of post-sleep grogginess and disorientation,” Pollak says.
—Donna Wilkinson, “Napping may actually help do the job for some people,” The San Bernardina County Sun, June 18, 1988

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