Introduction

Humans are subject to numerous rhythms during the year (Hildebrandt, 1966; Aschoff, 1981a, b), which impinge to various degrees on their physiology. Some of these are related to environmental temperature or nutrition and are dealt with elsewhere; the present review is concerned with the effects of daylength solely as illumination that acts through the eye on the central nervous system. For various reasons, most of the experimental evidence about the mechanisms involved in the control of rhythms by light has been obtained from studies on animals other than humans. In the following account some of this evidence will be presented and the extent to which it applies to humans will be considered.

Annual and daily rhythms

There are at least two kinds of rhythm that can be influenced by lighting: one that occurs over a day, such as motor activity, and the other over a year, such as breeding. Both depend on structures in the brain and there is reason to believe that yearly rhythms depend at least in part on daily rhythms. The daily rhythms (i.e. nychthemeral rhythms of precisely 24 h) persist as circadian rhythms (i.e. with a period of about 24 h) even in the absence of clues about the passage of time, and a change in the timing of events (Zeitgeber) will reset the rhythm, as happens when humans move from one time zone to another.

Subjecting animals that are seasonal breeders to progressively increasing daylengths in an otherwise constant environment will either induce, or suppress, the breeding season depending on the species.