IMPORTANCE OF 6 PM IN HAMSTER TIMEKEEPING EVIDENCED BY COMPUTER-ANALYSIS OF WHEEL-RUNNING ACTIVITY

We addressed the question whether the clock signal for hamsters to bec ome active occurs at sundown throughout summer or at some constant tim e after noon (p.m. time). Ten female golden hamsters housed in wheel c ages in a windowless room were exposed to 24-h light/dark (LD) cycles simulating the equinoxes (LD 12:12), when the sun sets at 6 p.m. and r ises at 6 a.m., and summer (LD 14:10, 16:8, and 18:6), when the sun se ts after 6 p.m. and rises before 6 a.m. The onset of behavioral estrus , a mask-free phase marker of the same clock that controls wheel-runni ng, was observed every 4 days, and wheel revolutions were recorded eve ry 5 min for 52 days. Computer analysis of the 5-min values averaged f or all 10 hamsters revealed a clear onset of running for each LD expos ure. Time in the windowless room is referenced to mid-L (room ''noon'' ) of the LD cycles. Although L-off ranged from 6 p.m. in LD 12:12 (6 h after mid-L) to 9 p.m, in LD 18:6, estrus began close to 4 p.m. and r unning close to 6 p.m. in every LD cycle. In a second study, 13 female s not tested for estrus began running closer to 7 p.m. in LD 16:8 (L-o ff, 8 p.m.), but when L-off was advanced to 4 p.m. they also began run ning on that day at 6 p.m. Testing for estrus may have made the first group of hamsters less fearful of light and therefore more responsive to a 6 p.m. clock signal to become active. It is conceivable that thes e nocturnal rodents voluntarily suppress, to varying degrees, overt ac tivity from 6 p.m. standard time to sundown to avoid predators. It is noteworthy that 6 p.m. room time also marks the onset of the clock's 1 2-h light-sensitive period underlying hamster timekeeping.