Arrhythmic — this term is called a person about whom it is impossible to say exactly who he is — «owl» or «lark». The wakefulness mode of the arrhythmic is not tied to the morning or evening. An arrhythmic is equally productive (or not very productive) can work both in the morning and at night.
There are observations that most of the most healthy arrhythmic people after some time (from a year to ten years) fall into serious depression. More precisely, they are statistically significantly more likely to fall into depression than larks.