Why is it that some people prefer to stay up until late in the evening, and others go to bed early? Do you consider yourself an early bird or a night owl?
Cultures have an obvious impact on our sleeping habits. Early wakings are in Western cultures associated with (work) efficiency. Traditionally, work has begun in the morning. In this way, culture influences our sleep rhythms to some degree.
However, not only culture and bad habits are thought to influence sleep rhythms. Genes seem to influence day time preferences as well. A review by Von Schantz & Archer (2003) looked a bit further at the role of genes, and the sleep differences among humans.
The functions of sleep are many. The sights of humans work most efficiently during the day, and in an evolutionary perspective, sleep increases food availability and improves survival.
Many predators are active during night time, and for this reason, it was more safe for us (the prey) to gather and hunt for food during the day time in the human prehistory. In animals studies, safety at the sleep place has been linked to longer sleeps (Acerbi & Nunn, 2010).
Melatonin is a hormone that influences our sleep-wake cycle, and it is regulated by light. In animals, it is responsible for a number of seasonal functions such as reproduction. Increased levels of melatonin has been found to promote earlier sleep onset and morning awakening (Eagles, 2009).
Melatonin is regulated by light that is processed by our sense of sight, or more specifically, the occipital lobe. Our sleep rhythms are therefore determined by the occipital lobe as well.
On some of the Micronesian islands, a high number of color blind people live, and because they completely lack the ability to see colors, they are not affected as much as other individuals by melatonin. Therefore, a relative large amount of the population specialize in night fishing.
Evidence for the role of genes has been found in a large family, who suffered from an advanced sleep phase disorder that resulted in abnormal melatonin levels, temperatures, and sleep rhythms due to a mutation of the Per2 gene.
However, the gene that has been found most consistently to alter day time preferences is Per3. Von Schantz & Archer (2003) concludes that their animal study of mice is not entirely reliable or applicable to humans. The reason for this is that the mice are studied under extremely controlled conditions, and humans do not live like that.
Humans are influenced by culture, habits and other factors that simply cannot be accounted for in animal studies. Furthermore, a specific gene was moved from the mice, but instead it should have been altered, since this is more likely to reflect the genes in humans (gene mutations).
The human sleep-wake rhythm is probably polygenic as the alteration of a number of genes (e.g., Per2/Per3) have an impact on sleep. Taken together, genes may explain some sleep disorders and why sleepiness occurs extremely early in some and extremely late in others.