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

Example Citation:
"'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

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