Why does oversleeping make you tired?


Why does oversleeping make you tired?

This question came from a reader submission (thanks Rodrigo!). Has a curious question stumped you lately? Feel free to submit your own question here:


📓 The short answer

Oversleeping can make you feel more tired because it throws off your internal clock. You might wake up during a deep sleep stage, miss your body’s natural cortisol boost that helps you feel alert, and wake up dehydrated – all of which can cause you to feel tired.

📚 The long answer

After a long week of early alarms, nothing seems as tantalizing as sleeping in way past your usual wake-up time on the weekend.

But the plan backfires: After more sleep than usual, you feel ... tired?

Here are a few reasons why oversleeping (i.e. sleeping past your normal wake-up time) leads you to feeling more tired:

Reason #1: Your circadian rhythm gets messed up.

Your body has a built-in 24-hour internal clock called the circadian rhythm. Largely regulated by light exposure, this clock regulates when you feel awake or sleepy, along with numerous other bodily mechanisms like metabolic activity, heart rate, body temperature, and hormone secretion.

Evolutionarily, the circadian rhythm provides several advantages. By feeling awake during the daylight hours, we'll have a better chance at finding a mate, avoiding predators, and collecting food.

When you sleep in, your body figures it's still nighttime because your eyes are closed. Darkness = night as far as your circadian rhythm is concerned, so this pushes your body out of sync with its usual schedule (similar to jet lag). This misalignment throws off your internal clock and can lead you feeling more fatigued.

Reason #2: You may be waking up in a deep sleep stage.

When you sleep, your body goes through multiple repeating cycles of different stages:

  • Stage #1: Light sleep: This is when you're drifting off to sleep. Your muscles relax and your heart rate slows.
  • Stage #2: Light sleep (but deeper): Your body temperature starts to drop, heart rate slows more, and eye movement stops.
  • Stage #3/4: Deep sleep: This is when your body enters a restorative state, repairing tissue and strengthening the immune system. Your breathing rate and heart rate slows, and your brain enters a slow/delta waves pattern.
  • REM sleep: In "rapid eye movement" sleep, your brain activity increases to levels nearing those seen when you're awake. Your body paralyzes itself, with the exception of breathing and eye movement. This is when you're dreaming.

A full sleep cycle goes through each of these stages and lasts about 90 minutes. You repeat this cycle about 4-6 times while you're asleep.

Oversleeping increases the chance you’ll wake up in the middle of a deep sleep stage, rather than at the end of a cycle. Waking up from deep sleep can trigger sleep inertia, a period of grogginess and mental fog that occurs after abrupt awakening. While we all experience some degree of sleep inertia, it's most intense after you wake up during deep sleep.

Reason #3: Hormones that signal sleepiness and wakefulness get thrown off.

Hormones play a large role when and why you feel sleepy or alert. Two of the most critical hormones in this function are melatonin and cortisol.

Melatonin signals to your body that it's time to go to bed and ramps up in the evening, peaking in the middle of the night before dropping back down to daytime levels in the early morning.

Cortisol, often called the "wake-up hormone," is a stress hormone that signals it's time to get up. It peaks shortly after your normal wake-up time before dropping down to lower levels throughout the day.

Melatonin and cortisol are designed to move in opposite directions. When melatonin is high and cortisol is low, you feel sleepy. When cortisol is high and melatonin is low, you feel awake. Oversleeping disrupts this hormonal balance.

When you oversleep, you may miss the cortisol peak that helps you feel awake. So even though melatonin is tapering off, you're waking up without the alertness boost that cortisol provides. The result is you feel sleepy even though you got extra sleep.

Reason #4: You may be dehydrated.

While you sleep, you lose water from sweating, waking up to urinate, and breathing (especially if you're a mouth-breather). Because you can't replenish your fluids while you're asleep, it's common to feel dehydrated when you wake up.

If you sleep more hours than usual, it's more likely that you're dehydrated and this can cause a stronger feeling of fatigue than if you had woken up earlier.

---

Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :)

All my best,

Sources for this week's newsletter



🌐 Wikipedia article of the week

Poe Toaster

"Poe Toaster is ... an unidentified person (or probably more than one person in succession) who, for several decades, paid an annual tribute to the American author Edgar Allan Poe by visiting the cenotaph marking his original grave in Baltimore, Maryland, in the early hours of January 19, Poe's birthday. The shadowy figure, dressed in black with a wide-brimmed hat and white scarf, would pour himself a glass of cognac ... and raise a toast to Poe's memory, then vanish into the night, leaving three roses in a distinctive arrangement and the unfinished bottle of liquor. Onlookers gathered annually in hopes of glimpsing the elusive Toaster, who did not seek publicity and was rarely seen or photographed."


📚 Did you know all affiliate revenue from book purchases is donated to charity?

Today You Should Know is an affiliate for Bookshop, an online book marketplace that supports local, independent bookstores.

I frequently update the Today You Should Know recommended reading list with my favorite fiction and nonfiction books. And I donate all affiliate proceeds to Room to Read, a leading nonprofit for children's literacy and girls' education across Asia and Africa.

It's a win-win-win: you get to read a book, small bookstores get some business, and money is donated to a great cause.

📕 Latest addition: Animal Farm by George Orwell.


👀 Catch up on other curious questions

P.S. 🤔 Do you have a question for the newsletter? Click here to submit your question (or reply to this email!)


Refer friends, earn rewards!

Have a curious friend who'd love to learn more about the world? Share the link below and you'll receive rewards for referring people to Today You Should Know.

📧 (1) Refer just ONE person and receive a special "Five Fast Facts" edition of the newsletter.

📣 (5) Refer five people and I will shout you out in the newsletter.

📚 (10) Refer 10 people and I will ship you a free, surprise book on an interesting topic.

Thanks for spreading the word!

[RH_REFLINK GOES HERE]

facebook twitter linkedin email

P.S. You have referred [RH_TOTREF GOES HERE] people so far.

💡 Today You Should Know

Learn something new every Friday. Join 2,000+ nerds and satisfy your curiosity one question at a time with topics including science, technology, history, and more.

Read more from 💡 Today You Should Know
fly rubbing hands together

Why do flies rub their legs together? 📓 The short answer Flies rub their legs together to clean off dust, dirt, moisture, and pollen that can interfere with their senses. Staying clean helps them fly, avoid infections, an taste with their feet. 📚 The long answer You've probably spotted a fly rubbing its little legs together and wondered what that's all about. The options run through your head (memes linked for illustrative effect): (A) Plotting world domination (B) Gearing up for a delicious...

U.S. Capitol

Longtime readers of Today You Should Know may recognize this question – it's a repeat! I just returned from a fantastic trip seeing lovely people and climbing beautiful mountains. Not one word was written as I focused on being present. Hope you enjoy (re)learning about why the U.S. eats quarter pounders and not ... 113 grammers 🍔. Back next week with a new curious question. 👋 Caitlin Why doesn't the U.S. use the metric system? 📓 The short answer Pirates are a big reason why the United States...

man shaving himself

What does shaving cream do? This question came from a reader submission (thanks Saurav 💘!). Has a curious question stumped you lately? Feel free to submit your own question here: Submit your question 📓 The short answer Shaving cream is a specially designed soap that forms a dense, long-lasting foam. It lifts hairs for a smoother, closer shave, hydrates and protects your skin, and cleans away oils and debris as you go. 📚 The long answer Humans have been shaving for thousands of years, but the...