Graded changes in light intensity at dawn and dusk are a hallmark of the natural illumination cycle, yet their contribution to circadian entrainment has received little experimental attention. The program described below will make use of recently developed twilight simulation technology, for systematic investigation of the role of dawn/dusk transitions in the entrainment of circadian rhythms of nocturnal fossorial rodents (rats and hamsters) and of diurnal primates (squirrel monkeys)--animals adapted to vastly different lighting environments. In keeping with our long-term goal of elucidating the process of photic entrainment under naturalistic, ecologically valid conditions, the rodents will be provided with a dark nest compartment and will thus have the opportunity to self-select their daily light exposure. Experiments are designed to test a number of specific hypotheses, formulated on the basis of preliminary evidence from our laboratories and those of others.
We aim (1) to examine entrainment and light self-exposure patterns under twilight cycles with seasonally-varied photoperiods and with different cycle lengths, and under corresponding rectangular light-dark (LD) cycles; (2) to compare the zeitgeber strength of these two cycles, as well as any after-effects they might exert on circadian period measured in constant darkness (DD): (3) to identify distinctive features of the self- selected daily illumination pattern and determine their contributions to period and phase control; (4) to evluate potential light adaptation effects occurring during the early portion of the dawn twilight signal; and (5) to analyze the temporal relations between the circadian rhythm of visual sensitivity, as measured by psychophysical signal-detection methods, and dawn/dusk illumination cycles. Recent observations in winter-depressed patients suggest that low-intensity twilight signals can have marked therapeutic consequences, and elicit shifts in sleep phase and in melatonin secretion patterns previously thought to require bright-light interventions. This, in seeking a systematic account of twilight-induced circadian effects in both nocturnal and diurnal mammals, within the present proposal, there is already a strong basis for expecting that refined clinical treatments of sleep phase and seasonal affective disorders will ensure.
| Boulos, Ziad; Macchi, M Mila (2005) Season- and latitude-dependent effects of simulated twilights on circadian entrainment. J Biol Rhythms 20:132-44 |
| Boulos, Ziad; Macchi, M Mila; Terman, Michael (2002) Twilights widen the range of photic entrainment in hamsters. J Biol Rhythms 17:353-63 |
| Boulos, Z; Macchi, M; Terman, M (1996) Twilight transitions promote circadian entrainment to lengthening light-dark cycles. Am J Physiol 271:R813-8 |
| Boulos, Z; Macchi, M; Houpt, T A et al. (1996) Photic entrainment in hamsters: effects of simulated twilights and nest box availability. J Biol Rhythms 11:216-33 |
| Boulos, Z; Terman, J S; Terman, M (1996) Circadian phase-response curves for simulated dawn and dusk twilights in hamsters. Physiol Behav 60:1269-75 |
| Boulos, Z; Macchi, M; Terman, M (1996) Effects of twilights on circadian entrainment patterns and reentrainment rates in squirrel monkeys. J Comp Physiol A 179:687-94 |
