The mantra of 7-8 hours of sleep is based on large population studies. Individuals vary in sleep requirement and this is a function of:
- How efficient your sleep is: Deep slow wave sleep (Stages 3-4 NREM sleep) is the most restorative. Frequent interruptions to sleep – say due to obstructive sleep apnea or a noisy environment – leads to a reduction in time spent in deep sleep. You may not even be aware of the brief awakenings that interrupt your sleep. In such cases, you may wake up thinking that you have been sleeping for 7-9 hours when in actual fact you have only slept 60-70% of that time or less.
- How much sleep you have had in the previous 24 hours: The more sleep deprived you are, the more sleep you will need.
- Your genetic makeup: Some individuals seem to manage very well with a reduced sleep of 6 hours or less.
- Other conditions you may have: If you have an infectious disease (like flu) you may find that you are sleepy and need to sleep longer. This may be explained by the production of inflammatory mediators that act as hypnogens in the brain. Any condition that disturbs sleep at night will also cause a compensatory increase in time spent in bed to compensate for reduced sleep efficiency (as seen in point 1 above).
So, given that sleep needs can vary in the same person at different times and can also vary between individuals, how do we find out how much sleep we need. Unfortunately, there is no blood test to determine this. The simplest way to determine your sleep need is to spend 5-10 days free sleeping (that is, start your sleep at the same time each day and then stay in bed as long as you need, to feel restored and fresh the next morning). Over time you will settle into a pattern that will show you how much sleep you actually need. If this is inordinately long (> than 9 hours) you should seek medical advice for possible conditions that may be preventing you from sleeping efficiently.