hormones such as adrenal steriods. If the daily pulsatile release of either or both are shifted, the animals may shift from a tendency to lipid storage to one of lipid utilization and increased migratory activity. Experiments in birds in which such phenomenon have been described have also shown that a single pulse of light during the night of a normally short day may cause the appearance of a new pulsatile release of pituitary hormone. However, in most cases, one would expect the physiological rhythm to be "reset" to normal phasing during the subsequent day. Thus, unless the nighttime interruptions were frequent, only minor consequences would be expected. 2. Photoperiodism and endogenous circadian rhythms: Perhaps the most potentially damaging aspect of abnormal environmental lighting would be its effect on daily timing processes that control long-term (e.g., annual) biological rhythms. Photoperiodism, the dependence on the length of the day to regulate various annual rhythms, is well established for a variety of seasonal physiological activities (e.g., growth, reproduction, molt, migration and diapause) in a diversity of animals, including fish, amphibians, reptiles, birds and mammals. It is important to recognize that a photoperiod change of only about 5-30 minutes may be sufficient to elicit a change in the physiological status of animals. The important aspect of this photoperiodic phenomenon in the present context is that the underlying time measurement mechanism is based on an endogenous circadian rhythm in photosensitivity. Thus, while the timing of seasonal cycles is normally dependent on the average daily photoperiod in different seasons, the circadian rhythm underlying the measurement of this photoperiod often depends on aspects of the daily light profile other than simply the total number of hours of visible light per day. In particular, experiments have shown that in most photoperiodic species
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