This seven hour rule appears to be true in most cases. However, I cannot help but wonder why I feel much more rested when I sleep for eight hours than sometimes when I sleep only for five hours. Clearly the numbers alone do not tell the whole story about the importance of sleep to our health.
Recently some scientists and other public figures have espoused the notion that modern generations are not sleeping as much as we used to due to the ubiquity of conveniences such as artificial light and light-emitting electronic devices today. In fact, some studies show that light exposure from nightly iPad usage disrupts sleep. Many more show the detrimental impact of sleep deprivation on human cognitive ability and mood. As a result it is believed that overexposure to light results in lack of sleep and can contribute to the recent rise in neurological and mental health disorders such as Alzheimer’s disease and depression.
Contrary to this belief that disruptive light causes us to sleep less, a new study published in Current Biology argues that individuals in hunter-gatherer societies may sleep even less than we do. Scientists analyzed the habits of modern foragers in societies in Tanzania, Namibia and Bolivia. Results showed that these individuals sleep for an average of six and a half hours per night. This is easily comparable to the average seven hours of sleep people in industrialized societies receive. In fact, a significant portion of these hunter-gatherers sleep for less time than this, which indicates that lack of sleep may not be the only explanation for the surge in health problems related to disruptive modern light environments.
For a long time scientists and clinicians have assumed that destructive light schedules such as those from night shift work, jet lag and artificial lighting can cause problems in mood and learning by disrupting circadian rhythms and sleep. However, Hopkins scientists demonstrated that aberrant light itself is sufficient and independent from sleep and circadian rhythms in causing learning impairments and behavior similar to depression in adult mice. As a result, we can conclude that light itself has a major impact on our health.
Return now to the hunter-gatherer study. Why might the hunter-gatherers be healthier despite sleeping less than those living in the modern world? Although some might point to the fact that modern urban generations have grown more sedentary, the study surprisingly found similar levels of daily physical activity in the hunter-gatherers’ and individuals living in the modern world’s lifestyles.
Thus the most plausible explanation is the difference in exposure to light. While hunter-gatherers are exposed only to the natural light environment, we are continuously exposed to artificial light even after the sun has set. Due to the direct impact that light has on cognition and mood, this overexposure to light may have caused increased depression and learning problems in spite of getting more sleep! In other words the problem may not be that we are not getting enough sleep. The real problem is that we are turning on the light and playing on our iPhones too much at night.