1. Fetal

The fetal position gets its name from the shape of a baby in the womb. In the fetal position, a sleeper’s body is essentially curled up into a ball as they lie on one side with legs and arms bent. This position is commonly assumed by adult sleepers. Exact numbers vary, but studies show that on average, over 60% of adults spend the majority of the night in a side position. Older age and a higher body mass index are both associated with more side sleeping.

In the 1970s, sleep researcher Samuel Dunkell was the first to hypothesize that the fetal position corresponded with certain personality characteristics. Through his research, Dunkell observed that fetal position sleepers tend to be more anxious and emotional. This was in contrast to the semi-fetal position, whose adherents he described as being well adjusted.