The subject of sleep debt is not one that scientists agree about. It is known that getting too little sleep-time has detrimental effects on the brain, on the immune system and on metabolic function. Exactly how much is too little is still unclear.
Most experts suggest that 6-8 hours is sufficient. But there seem to be individual differences that allow some people to get by on less. There also seem to be individual differences that cause other people to need more.
Some studies suggest that it is not the total amount of time spent sleeping that is most important. A number of studies have been conducted in recent years concerning the connection between shift-work and cancer.
A person that works from 11pm to 7am may still be able to get 8 or more hours. But those hours will be daytime hours.
Melatonin production is always higher after the sun goes down. It is one of the biological chemicals that make a person sleepy.
Shift work does not necessarily cause a sleep debt, but it does disrupt the hormone production and that damages a person’s health over time. In 2007, the International Agency for Research on Cancer concluded that shift work is probably carcinogenic to humans.
So, we know that sleeping at night is important for good long-term health. But, we still have questions about the ideal amount of time to spend sleeping.
When it comes to daytime performance and mood, things like being able to drive or complete complex projects without becoming irritable, scientists refer to it as neurobehavioral functions. To date, only one study has compared the effects of total lack of sleep to sleep debt.
The researchers concluded that sleeping less than six hours per night for two weeks resulted in a loss of neurobehavioral function equivalent to what was seen after two nights of complete wakefulness. Some of the study participants were allowed to sleep four hours, while others were allowed to sleep six. There was little difference between behavior and function in those two groups.
From the results of that study, we can surmise that sleeping for six hours or less per night for days on end will cause us neurobehavioral problems. One thing that caused some concern among the researchers is that the volunteers were largely unaware of the problems caused by the growing sleep debt.
The researchers used scientific tests to gauge neurobehavioral function. They also asked the volunteers to rate their level of sleepiness. For the most part, the volunteers did not feel that sleepy, nor did they feel they were having difficulty performing the required tests.
In other words, you could be walking around with a growing sleep debt and not even be aware of it. This could affect your ability to perform complex tasks. Luckily, you can recover.
Several studies have shown that the idea of “catching up on your sleep-time” is valid. Your body will quickly recover. Just one day of sleeping 8 or 10 hours can clear your brain of any fogginess caused by sleeping for less for days. But you may need to pay back your sleep debt by getting extra sleep over several days.
If you normally need 8 hours of sleep but miss 2 hours you don't just need to sleep an extra 2 hours to get out of sleep debt – it will take more. This is because the amount of sleep you need is related to the number of hours you are awake. When you sleep 8 hours you are awake 16 hours (1 hour of sleep needed for 2 hours of being awake, a 1 to 2 ratio).
So if you only got 6 hrs sleep you'd be awake for 18 hrs. Since you are awake for 18 hours you really need 9 hrs of sleep according to our 1 to 2 ratio. That means if you got 6 hrs sleep your sleep debt would be 3 hrs and not just 2.
Much of the evidence about long-term health problems related to sleeping less than 6 hours is contradictory. There are observational studies linking shorter sleep-times to everything from diabetes to obesity. But, there is no clear data supporting the idea that sleeping less actually caused those health problems.
When it comes to mood, depression has been associated with insomnia and with sleeping more than 8-10 hours per night. High blood pressure, heart disease and slow healing could be caused by a chronic sleep debt. Or the other health problems could cause an inability to stay asleep. We just don’t have all of the answers, yet. The best suggestion is to aim for 6-8 and take a nap when you need to.