A: From time to time, many of us experience a dragging feeling of fatigue and grogginess when we awake from a deep sleep. Scientists call this "sleep inertia."

As the name implies, sleep inertia is a condition where the brain has not yet shifted into a higher cognitive gear, and performing even simple tasks such as tying your shoes or brushing your teeth can be a challenge.

There are two kinds of sleep, explains Steve Weber, head of sleep diagnostics at UW Hospital. There is a deep non-rapid-eye movement condition that tends to manifest itself when we first fall asleep at night and just before we awake in the morning - and also when we overindulge in sack time.

The second kind of sleep is rapid-eye movement or REM sleep, which involves dreaming and occurs after we slumber through about 90 minutes of deep sleep.

Sleep inertia occurs, typically, when we awake from the deeper, non-REM kind of sleep, says Weber.

"If you oversleep, there is a tendency to experience this non-REM type of sleep," he adds. "Then it takes a while - about 45 minutes - for your brain to get all of its neural networks into operation."

That muddleheaded feeling, he says, represents the "persistence of the sleep networks in the brain."

- Produced in cooperation with University Communications

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