What is sleep debt?
Many sleep researchers believe that modern society suffers widespread chronic sleep deficit. Others aren’t so sure.
There’s a folk understanding of “sleep debt”, when people haven’t had enough sleep. The unarticulated idea is that this is somewhat like a financial account, and that a built-up debt must be repayed. Indeed, people do sleep longer on weekends than on weekdays. But the popular understanding of sleep debt is not wholly accurate.
For starters, it is not a straight balance. Even after serious sleep deprivation, most people usually need only two or three good nights’ sleep to get back to normal. If you run short one hour per night for 5 nights in a row (say Sunday through Thursday), you don’t necessarily need 5 extra hours on the weekend.
Another exaggeration or misconception, according to some dissident sleep experts, is the idea that we as a society get a lot less sleep than we used to.
Some interesting studies
Jim Horne of Loughborough University points out that intelligent animals such as bears and tigers sleep much more in captivity than in the wild. Since it is unlikely that they are sleep-deprived in the wild, other factor(s) must account for their sleepiness when penned up, such as boredom.
Horne, who is one of the foremost sleep researchers in the world, compares the situation to hunger. Asked if they are hungry, people may say no – but the same people in the same situation in a restaurant with the smell of food and other people eating – those same people will feel hungry. Context matters a lot
Similarly, people may not feel particularly sleepy near their regular bedtime, but once they lie down may fall asleep within a normal period of time. Similarly, the Inuit (Eskimos) traditionally spent the long Arctic winter nights by sleeping for 14 hours a day, compared with around six hours daily during the continuous summer light. Despite the eight-hour seasonal difference, the Inuit did not suffer from sleep deprivation in the summer, as the key sign of insufficient sleep – excessive daytime drowsiness – was absent. Now that they have adopted a modern lifestyle, Inuit people tend to sleep about the same length of time as other people.
In a recent article in the Journal Sleep, Horne argues that sleep debt is largely a myth and that the average healthy adult needs only 7 to 7.5 hours of sleep per day. He suggests that most people would get more out of a 15-minute afternoon nap than an extra hour of sleep at night.
Horne told the BBC: “The test of insufficient sleep is whether you are sleepy in the day or if you remain alert through most of the day.”
David Dinges of the University of Pennsylvania raises objections to Horne’s arguments, although he concedes that there is little hard scientific evidence on sleep debt in the general population. Indeed, sleep debt experiments have really been sleep deprivation experiments and with a limited number of participants. Also, they measure the effects of sleep deprivation through cognitive tests designed to measure mental performance. It is not clear that everyone needs to be at peak mental conditions all the time.
Total vs partial sleep deprivation
There are differences in how the body reacts to total sleep deprivation (no sleep) and partial sleep deprivation (substantially restricted sleep over the course of consecutive nights.) Both reduce a person’s mental and physical abilities. Dinges estimates that about 4 or 5 nights of partial deprivation (at 4 hours sleep per night) reduces cognitive abilities as much as one night of total sleep deprivation.
Dr. Dinges estimates the chronic (on-going) short sleep of 6 hours or less per night makes a person as mentally restricted as 2 consecutive nights of sleep deprivation. Dr. Horne believes otherwise and doubts that sleep debt ever gets this bad in everyday life or that short sleep produces as negative effects as others assert.
Some experts (including economists) are even beginning to wonder if widespread sleep deprivation is having an effect on society’s brainpower and creativity. They are advocating that sleep deprivation be recognized with the same seriousness that is associated with the impact of alcohol as a public health problem.
Total sleep deprivation reduces the function of the parts of the brain involved in higher order thinking and cognition. Tests with advanced imaging systems show less energy use in the frontal lobe and thalamus. These are key areas of the brain involved in attention, alertness, and decision-making.
After one night of recovery sleep, the metabolic levels in these brain areas increase, but only partially. It takes more than one night (8 hours) of recovery sleep for the brain to fully recover from total sleep deprivation, as measured by consumption of glucose and oxygen. It is reasonable to assume that functionality (IQ, etc.) is also depressed after only one night of recovery sleep. Slow-wave sleep seems especially important for the frontal lobe to maintain optimum function.
Do you really have sleep debt?
People who claim they are suffering sleep debt often show no more daytime sleepiness than anyone else. This has been shown in a study using the MSLT and PVT tests. People might say they have sleep debt, but there is no objective external sign. And formal diagnosis of insomnia requires that the subject experience excessive sleepiness during the day to truly have clinical insomnia. This disconnect may reflect our general ignorance about our own sleep patterns and what happens at night. Sleep is a mystery to even the sleeper. But it argues against reports of sleep deficit being as prevalent as some believe.
It is fairly clear that there can be such a thing as “REM debt”. When people are selectively woken at the start of their REM periods a repeated number of times, they spend a lot more time in REM when they are finally allowed to sleep uninterruptedly. REM sleep loss causes hyperalgesia – increased sensitivity to pain.
Even animals in hibernation sleep. Periodically during the hibernation stage, the animal will come out of hibernation in order to sleep. This means that a sleep debt builds up in the animal’s brain even during hibernation.