After a week of waking at 5:45 am to a blaring alarm, the idea of a leisurely Saturday morning can seem downright therapeutic.

So why the heck do you wake at 10 am feeling as if some cruel person stuffed eight thousand cotton balls into your skull?

Sleep experts have a name for this frustrating phenomenon.

They call it sleep inertia, also known as sleep drunkenness.

“It’s really more about sleep timing than sleep amount,” says Chris Winter, MD, one of the world’s top sleep doctors and co-architect of Precision Nutrition’s Sleep, Stress Management, and Recovery coaching. 

“All of the brain’s timing cues have been disrupted.”

Among those timing cues:

• The blaring noise of your alarm clock that triggers the release of cortisol and other alertness chemicals
• Overhead and outdoor light that sets your brain’s circadian clock
• Conversations with housemates that nudge you to “wake up! think!”
• Caffeine
• Breakfast
• That feeling of being rushed as you gather up your things and race out the door

When you sleep in, you deprive your brain of those cues.

Some of the wakeup signals don’t take place at all, such as the alarm and that mad dash out the door.

Others, like overhead lighting and caffeine, take place hours later than your brain is used to getting them.

End result: You don’t get as big of a dose of alertness chemicals as you’re used to.

It gets worse.

Because you slept in, you likely won’t feel tired at your usual bedtime. 

Most people need to be awake for about 16 hours before they feel sleepy. If you wake at 10 am Saturday morning (instead of 5:45 am), you’ll probably stay up later than usual.

And that means you’ll most likely compensate by sleeping later on Sunday morning, which means you’ll probably struggle to fall or stay asleep Sunday night.

Which gets us to Monday morning.