Circadian rhythms are also generated by endogenous clocks. Loher (1989) studied the calling rhythms of crickets and showed that these internal clocks are reset by the light dark cycles of day and night. This figure shows an ‘actogram’ of a male Teleogryllus. Each horizontal line is a day in the life of the male and the vertical hatching represents when the male is active (calling). The upper portion of the figure shows that when the male is exposed to a light-dark cycle (LD), the male calls exclusively during the evening and the cycle shows a typical 24 hour period. When the male is then kept in a continuous light (LL) the calling activity still shows a rhythm but with a period longer than 24 hours. Thus, the constant drift in the daily pattern shown in the bottom portion of the actogram. In natural conditions the endogenous clock is reset each day by the cycle of the sun.