Answer by Nipun Sher:
Situation: You are so drowned in school or office work that you hardly get time to sleep on weekdays. You tell yourself that you will make up for the lack of sleep (sleep debt) on weekends. In fact, this promise of make-up sleep is one of the things that keeps you going. The much-awaited weekend comes and you try to “catch up” on lost sleep by sleeping longer hours, only to find out that you feel even more tired after waking up than sleeping in. Instead of feeling refreshed and rejuvenated, you feel sluggish and incapacitated.
I bet everyone has faced such a situation at one point of time or the other. So, what’s going on? And, why does it happen?
Big ol’ bum aka Body clock | Image Source:
Basically, your body clock is a big ol’ bum. It has a penchant for following a regular routine (). When you oversleep, you mess up this routine. Big ol’ bum doesn’t like it and just goes bonkers. This results in a situation where your internal clock and body are not in sync. Due to this disruption in rhythm, your sleep cycle goes haywire and you end up feeling groggy and fatigued. This is very similar to how you feel when you are jet lagged.
Time for some definitions:
Sleep cycle: The sleep cycle lasts anywhere between 80-120 minutes, with the average being 90 minutes. Also, the average person has approximately 5 cycles every night, which amounts to a total of 7.5 hours.
Stages of Sleep Cycle | Image Source:
Now, when you oversleep, you extend the number of sleep cycles. Generally, when you wake up, it is usually in the middle of a cycle. If it is during deep sleep or the REM stage of the cycle, you can end up feeling lethargic and spent. Additionally, the quality of sleep matters, too. If you spend more time in Stage 1 (lightest sleep), you will end up having a poor-quality sleep overall. As a result, you would try to compensate for the lack of quality with increased quantity, i.e., you will tend to oversleep.
Sleep debt: It is the deficit in sleep caused by the lack of enough sleep.
From Scientific American:
Sleep debt is the difference between the amount of sleep you should be getting and the amount you actually get.
Sleep inertia: A body in sleep will want to stay in sleep unless an external force, such as Mom’s kick in the tush, is applied to it.
Jokes apart, sleep inertia may result in making you feel groggy after you’ve overslept. Here’s the definition from a research study on it:
Sleep inertia is the impaired cognitive performance immediately upon awakening, which decays over tens of minutes. This phenomenon has relevance to people who need to make important decisions soon after awakening, such as on-call emergency workers.
Oversleeping, it is believed, may cause you to experience sleep inertia more often. It is due to this, you feel the need to go back to sleep after waking up.
This state of sleep inertia, if severe enough, can also lead to a condition called sleep drunkenness or confused arousal.
Sleep drunkenness (SD) consists of difficulty in coming to complete wakefulness accompanied by confusion, disorientation, poor motor coordination, slowness, and repeated returns to sleep.
A study  published in the Neurology journal found that nearly 15% of us, or 1 in 7 people, suffer from sleep drunkenness. This “drunken stupor,” in severe cases, can also result in violent reactions and amnesia.
Oversleeping, if done once in a while, doesn’t result in any health risks; however, if you find yourself habitually oversleeping (), it can cause serious health issues. A study done by Harvard  found that those who slept between 9-11 hours every night were more likely to develop a heart condition and/or develop some memory-related issues. Some other studies  have linked long-term oversleeping to diabetes, obesity, and even early death.
Even if you sleep regularly for 7-8 hours every night, you may still wake up feeling out of sorts and unrefreshed. This could be due to some underlying disorders:
Obstructive sleep apnea: It is one of the most common sleep disorders. It causes people to stop breathing momentarily while they are sleeping. It can also lead to an increased need for sleep, as it disrupts the normal sleep cycle. Even if you get sufficient amount of sleep, you may still wake up feeling tired, as the sleep you got was probably pretty light and crappy.
Read more about it here:
Delayed sleep-phase disorder: People with this condition also tend to oversleep and wake up feeling groggy and tired.
Delayed sleep-phase disorder (DSPD), also known as delayed sleep-phase syndrome (DSPS) or delayed sleep-phase type (DSPT), is a circadian rhythm sleep disorder affecting the timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily rhythms, compared to the general population and relative to societal requirements. People with DSPD generally fall asleep some hours after midnight and have difficulty waking up in the morning.
Furthermore, people with conditions like, , also tend to oversleep, and thus, feel fatigued.
One thing I must mention that not everyone who oversleeps does so because of some underlying condition. Alcohol and some prescription drugs can also tend to cause people to oversleep.
Role of Genetics
We all have a friend who thrives even on little sleep. We also know people who need at least 9 hours of sleep to flourish. So, what the deal? Normally, people who sleep less than 6-6.5 hours a day are said to be short sleepers, whereas those who sleep more than 9.5 hours are considered long sleepers. Some studies have found that genetics also plays a big role in how much sleep one needs. A study  published in Science successfully identified a rare mutated gene, DEC2, in a mother-daughter pair that enabled them to function normally on 6 hours of sleep every night.
Personally, if I feel groggy after waking up, taking a 10-minute walk outside, or sitting in the balcony with the sun beating down on me tends to take care of it.
Bonus: University of Michigan’s developed an app called Entrain to help people reset their body clock by logging the amount and type of light they get throughout the day. I have heard from a lot of people that it actually helped them a lot with their sleeping problems. For more details, go here:
AsapSCIENCE: How Much Sleep Do You Need?