Researchers at Leiden University have discovered the biological reason for the difference between morning types (larks) and evening types (night owls), by observing a time difference in their bodies' daily temperature curve. This shows that the biological clock runs two hours earlier in morning types than in evening types. Up to now, it was assumed that personality differences were involved, for example laziness, extraversion or social behaviour. The research was carried out as part of a project funded by the NWO's Foundation for Behavioral & Educational Sciences.
Regular measurements of the daily temperature curve in both types of people under laboratory conditions showed that, biologically speaking, some people wake up earlier than others. In a 24-hour economy, it would be possible for these morning and evening types to work at times which suit them best. However, it should be noted that morning and evening types make up only a small part of the population. In most people, the biological clock runs somewhere in between that of morning and evening types.
The biological clock, which regulates our body's circadian (i.e. daily) rhythms, is located in two small cerebral groups of cells, the suprachiasmatic nuclei. These cells emit chronometric signals to the body so that it synchronizes to the time of day. Heart rate, hormone secretion and body temperature, for instance, are adjusted accordingly. The rhythm of the biological clock cannot easily be changed. Evening types who have to work early in the day can get used to this, but during holidays they revert to their evening type behaviour.
It is possible that there is a genetic basis for morningness and eveningness.