the first thing experts will tell you about sleep is that there is no "magic number." Not only do different age groups need different amounts of sleep, but sleep needs are also individual. Just like any other characteristics you are born with, the amount of sleep you need to function best may be different for you than for someone who is of the same age and gender. While you may be at your absolute best sleeping seven hours a night, someone else may clearly need nine hours to have a happy, productive life. In fact, a 2005 study confirmed the fact that sleep needs vary across populations, and the study calls for further research to identify traits within genes that may provide a "map" to explain how sleep needs differ among individuals.
Another reason there is "no magic number" for your sleep results from two different factors that researchers are learning about: a person’s basal sleep need – the amount of sleep our bodies need on a regular basis for optimal performance – and sleep debt, the accumulated sleep that is lost to poor sleep habits, sickness, awakenings due to environmental factors or other causes. Two studies suggest that healthy adults have a basal sleep need of eight to nine hours every night, but where things get complicated is the interaction between the basal need and sleep debt. For instance, you might meet your basal sleep need on any single night or a few nights in a row, but still have an unresolved sleep debt that may make you feel more sleepy and less alert at times, particularly in conjunction with circadian dips, those times in the 24-hour cycle when we are biologically programmed to be more sleepy and less alert, such as overnight hours and mid-afternoon. You may feel overwhelmingly sleepy quite suddenly at these times, shortly before bedtime or feel sleepy upon awakening. The good news is that some research suggests that the accumulated sleep debt can be worked down or "paid off."
Though scientists are still learning about the concept of basal sleep need, one thing sleep research certainly has shown is that sleeping too little can not only inhibit your productivity and ability to remember and consolidate information, but lack of sleep can also lead to serious health consequences and jeopardize your safety and the safety of individuals around you.
For example, short sleep duration is linked with:
- Increased risk of motor vehicle accidents
- Increase in body mass index – a greater likelihood of obesity due to an increased appetite caused by sleep deprivation
- Increased risk of diabetes and heart problems
- Increased risk for psychiatric conditions including depression and substance abuse
- Decreased ability to pay attention, react to signals or remember new information
According to researchers Michael H. Bonnet and Donna L. Arand, "There is strong evidence that sufficient shortening or disturbance of the sleep process compromises mood, performance and alertness and can result in injury or death. In this light, the most common-sense 'do no injury' medical advice would be to avoid sleep deprivation."
On the other hand, some research has found that long sleep durations (nine hours or more) are also associated with increased morbidity (illness, accidents) and mortality (death). Researchers describe this relationship as a "U-shaped" curve (see illustration) where both sleeping too little and sleeping too much may put you at risk. This research found that variables such as low socioeconomic status and depression were significantly associated with long sleep. Some researchers argue that these other variables might be the cause of the longer sleep: the fact that individuals with low socioeconomic status are more likely to have undiagnosed illnesses because of poor medical care explains the relationship between low socioeconomic status, long sleep and morbidity/mortality. Researchers caution that there is not a definitive conclusion that getting more than nine hours of sleep per night is consistently linked with health problems and/or mortality in adults, while short sleep has been linked to both these consequences in numerous studies.
"Currently, there is no strong evidence that sleeping too much has detrimental health consequences, or even evidence that our bodies will allow us to sleep much beyond what is required," says Kristen L. Knutson, PhD, Department of Health Studies, University of Chicago. "There is laboratory evidence that short sleep durations of 4-5 hours have negative physiological and neurobehavioral consequences. We need similar laboratory and intervention studies to determine whether long sleep durations (if they can be obtained) result in physiological changes that could lead to disease before we make any recommendations against sleep extension."
But a key question is how much is too much or too little. Researchers Shawn Youngstedt and Daniel Kripke reviewed two surveys of more than 1 million adults conducted by the American Cancer Society and found that the group of people who slept seven hours had less mortality after six years than those sleeping both more and less. The group of people who slept shorter amounts and those who slept longer than eight hours had an average mortality risk that was greater, but the risk was higher for longer sleepers. Youngstedt and Kripke argue that for those who would normally sleep longer than eight hours, restricting their sleep may actually be healthier for them, just as eating less than one’s appetite may be healthier in a more sendentary society.
