Sleep experts are quick to tell you there is no "magic number" when it comes to sleep.  Some people seem to get by fine on only a few hours per night, while others are an absolute wreck if they don't get nine or ten.  Unfortunately, there is also an increasing number of people who may think they are getting enough sleep, but are actually building up a sleep debt that has to be paid one way or another through reduced short and long term health, emotional instability, and poor performance.

The difference between basal sleep and sleep debt is important to understand, with individuals starting off with different base needs for sleep and also experiencing different levels of accumulated sleep debt.  While a number of studies have shown the average basal sleep needs of healthy adults to be between seven and eight hours per night, the interaction between this average and sleep debt can be hard to define.  There are also a number of people who lie on the outside of this range, with an individual's circadian rhythms directly affecting their sleeping needs.

The circadian clock actually controls your sleeping patterns, by controlling enzymes and inhibiting many of the processes associated with wakefulness.  Circadian rhythms also affect your sleep offset, which is why habitual 'early risers' have trouble sleeping in even when they are tired.  Along with individual differences in circadian patterns, some people also have a mutation of the DEC2 gene, meaning on average they sleep for two hours less than normal.

In one famous research paper by the University of California in San Diego, a study of over one million adults found that those people who lived the longest slept between six and seven hours each night.  There have also been multiple studies into links between sleep duration and particular conditions, with University of Warwick researchers finding lack of sleep to double the risk of death from cardiovascular disease.  According to Professor Francesco Cappuccio, "Short sleep has been shown to be a risk factor for weight gain, hypertension, and Type 2 diabetes, sometimes leading to mortality".

While not getting enough sleep is never a good idea, there has also been some findings that link too much sleep to increased mortality.  In the same study by the University of Warwick, mortality rates associated with long sleep patterns were not linked to potential physiological mechanisms, however, and were instead linked to psychiatric disorders like depression and bipolar disorder and cultural factors such as socio-economic status.

With the "magic number" for sleep duration so hard to define, individual people have to take responsibility for their own sleeping patterns.  It doesn't matter if you need a solid nine hours every night or are happy with six or seven, however, if you don't take care of your own basal sleep needs and pay off your sleep debt sooner rather than later, something will give.