(including invertebrates and vertebrates) interpretation of daylength is dependent on when the animal is exposed to light relative to its internal sensitivity rhythm. For example, an animal need not receive continuous illumination for 14 hours daily to "perceive" the photoperiod as being 14 hours in length. Rather, a relatively brief period of light (several hours) at the beginning of the day, followed by an even briefer pulse (an hour or less) centered at 14 hours after the beginning of the first pulse, may be sufficient to elicit the same response as 14 hours of continuous light. A common experiment of this type would involve exposure of an animal to a standard daylength (e.g., 6-12 hours), followed by an hour of light during the night (designated interrupted night experiment); such a photoregime would elicit physiological or behavioral responses characteristic of a "long" daylength. These results are especially relevant in the context of reflected light from the spacecraft: one must be concerned that illumination from reflections during nighttime hours might be sufficient to stimulate photoperiodic photoreceptors in such a way that an animal would initiate long-day processes in the middle of a season of short days (e.g., summer processes might be initiated in winter). The biological consequences of such untimely responses could well be disruptive and even lethal to an individual or population. Futhermore, experiments have demonstrated that such photoperiodic responses are sensitive to relatively low levels of illumination (1 ft. candle or less) and that even a single interruption of the night may be sufficient to initiate a physiological response, e.g., and increase in pituitary secretion of gonadotropin that is "intended" to initiate gonadal development. However, more regular daily interruptions would be required to maintain the accelerated long day response initiated
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