... with nonphotoperiodism in other laboratoryrodent species, relative to F344 rats. An alternative hypothesis, that differences are due to variation in melatonin secretion or responses to melatonin, ... genetically nonphotoperiodicindividuals in short photoperiod have a delayed, loweramplitude nocturnal rise in pineal melatonin, a 4.5 hourdelay in the onset of nightly activity, a longer free-runningactivity ... hamsters, the free runningrhythm is too short for proper entrainment to a short pho-toperiod, leading to an inability to produce a short pho-toperiod melatonin pattern [15]. In contrast, studies...