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Why Does Taking A Nap Make You Feel Groggy And Grumpy When You Wake Up?

Before you take your cat nap, be wary of sleep inertia!


Dr. Beccy Corkill


Dr. Beccy Corkill

Custom Content Manager

Beccy is a custom content producer who holds a PhD in Biological Science, a Master’s in Parasites and Disease Vectors, and a Bachelor’s in Human Biology and Forensic Science.

Custom Content Manager

Cat sleeping on the sofa at home. Happy tabby cat relaxing in a house.

Sleep inertia is something that can impact people that have rotating shifts or are on call regularly.

Image credit: Maliflower73/

Have you ever been longing for a nap all day, and once you have one, you wake up feeling terrible? Well, it is probably all to do with sleep inertia.

There are many benefits to a little cat nap in the middle of the day. It is known to reduce stress, improve our mood, boost the immune system, and well make you less tired. Having a sweet little snooze after lunch is thought to help us improve our work performance and stay alert.


There is also the fun-sounding "nappuccino", when people guzzle down a steaming cup of coffee just before a nap. Apparently, this little shot of caffeine can help people feel a bit more recharged when they wake up, as they have had a double whammy of sleep and caffeine.

Unfortunately for some, these little siestas just don’t work, and when they wake up they feel even sleepier and possibly even grumpier than they were before they took their little snooze.

Often, when we sleep we go through an array of different stages, there is rapid eye movement (REM) and non-rapid eye movement (NREM). NREM normally forms the majority of the sleep pattern, and even this we can divide it into light, intermediate, and deep sleep.

Typically, in a sleep cycle, we go from unconscious to NREM to REM (when we often dream). So why do we feel so groggy when we wake up from naps?


Well, it is something that is called "sleep inertia", and this can happen if your nap was too short or too long (you just need to get the nap just right like Goldilocks). The CDC defines it as “A temporary disorientation and decline in performance and/or mood after awakening from sleep," during which "People can show slower reaction time, poorer short-term memory, and slower speed of thinking, reasoning, remembering, and learning.” This is pretty much when your body does not want to wake up as it is deep into that NREM sleep. Some people even refer to it as “sleep drunkenness”.

Typically, this feeling does not last long, normally around 30 to 60 minutes, although some people may experience it for up to two hours.

It is unknown why sleep inertia occurs. But there are a few working theories:

  1. It may happen due to a little molecule called adenosine. This is something that has a build-up during the day and, when we sleep, decreases. It could be the reason why we feel so tired after a nap – it just has not had the time to be completely cleared yet.
  2. It may be caused by an increase in delta waves. These waves are more common in NREM and are likely to increase after sleep loss or deprivation. If your brain has not reduced the delta waves it could cause sleep inertia.
  3. During sleep cycles, there can be an increase or decrease in the blood flow to the brain. People that have chronic fatigue syndrome can have reduced blood flow, and this comes with symptoms similar to sleep inertia. There is a possibility that the blood reduction could be linked to sleep inertia.

Sleep inertia is something that can impact people that have rotating shifts or are on call regularly. This may decrease cognitive alertness at work and slow down reactions. In turn, this could increase work-related injuries and bad decision-making.


So how do we get over this?

"Power napping or napping for no more than 15 minutes prevents a person from getting into the deep stages of sleep that leave you feeling as though you are still half asleep after the nap," Dr Benjamin Nager, a neurologist at Northwestern Medicine Huntley Hospital, told POPSUGAR.

So maybe get a stopwatch on the go the next time you want to have a little snooze. 

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.   


All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.  


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