Modification by Light of 24-Hour Activity of White Rats

The 24-hour rhythm of running activity of white rats originates from a light-cycle and can be reversed by illuminating at night rather than by day. If continuous light is used, the activity phase of the rhythm remains 8-12 hours in length but shows a regular, constant and definite amount of alteration or delay in the time of starting so that the activity block travels around the clock. Behavior modification such as this should be called a photoperiodic response. In the present studies the photoperiodic data were: (1) the delay in time of starting activity for rats in continuous light was 3/4 hour per day, and their activity traveled around the clock in 16 days; (2) during the progress of a reversal of the 24-hour solar light cycle, when light fell on the activity block there was a regular delay of 3 hours/day, only until the activity block again fell in darkness; (3) finally with a 16-hour light cycle of 8 hours of illumination and 8 hours of darkness, the delay was continuous at 1 hour a day. By raising rats with no cycle of nursing or other regular environmental influences, further evidence was obtained for the inheritance of what seems to be an activity rhythm of about 12 hours of activity and 12 hours of inactivity