Siberian hamsters that fail to reentrain to the photocycle have suppressedmelatonin levels

Siberian hamsters readily reentrain to a 3-h phase delay of the photocycle (16 h light/day) but fail to reentrain to a 5-h phase delay. This study tes ted whether melatonin production was suppressed in animals that failed to r eentrain. Melatonin was measured on the day before, day of, or several days after each phase shift. Melatonin levels measured 4 h after dark onset wer e similar to 83 mu g/ml on the day before each phase delay and undetectable (<6 mu g/ml) during the light phase on the day of the phase shift. Activit y onsets regained their prior phase relationship to the photocycle 4 (3 h) or 5 (5 h) days after the phase shift; on that day, melatonin levels were m easured 4 h after dark onset. Melatonin levels were unaffected by the 3-h p hase delay (>57.6 mu g/ml) but were undetectable after a 5-h phase delay (< 8 mu g/ml). Thus melatonin remained suppressed only after the phase delay t o which hamsters also fail to reentrain. This relationship suggests that th e propensity for reentrainment may be influenced by changes in melatonin pr oduction following a phase shift of the photocycle.