Exposure to an Ambient Temperature outside the Appropriate Thermoneutral Range Chang- Es the Amount and Distribution of the Differ-

EXPOSURE TO AN AMBIENT TEMPERATURE OUTSIDE THE APPROPRIATE THERMONEUTRAL RANGE CHANGES THE AMOUNT AND DISTRIBUTION OF THE DIFFERENT stages of the wake-sleep cycle in several different species.1-3 These changes may be a consequence of the differences in the capacity to regulate body temperature across the different stages of the wake-sleep cycle.3,4 In particular, it has been shown that thermoregulatory responses like shivering, panting, and peripheral vasomotion are suppressed during rapid eye movement (REM) sleep5 due to a change in the activity of the central thermostat at the preoptic-hypothalamic level.6 In early studies carried out in the cat, a short-term exposure to different low ambient temperatures was shown to primarily affect REM-sleep occurrence and that the observed decrease was proportional to the ambient temperature of exposure, while morecomplex effects were observed on non-REM (NREM) sleep.1,2 With respect to the latter, “spindle sleep” was progressively depressed at ambient temperatures below 0°C, while slow-wave sleep was maximally decreased at an ambient temperature of 5°C but increased toward control levels as the temperature was lowered.1,2,7 This depression in sleep occurrence by the exposure to low ambient temperature has now been confirmed by many studies on different species.8-15 Exposure to low ambient temperature also clearly influences sleep occurrence when animals are allowed to recover at normal ambient temperature in the laboratory. In particular, a rebound of REM sleep, which was proportional to the degree of the previous REM sleep loss, has been observed in the cat.16 These results have been confirmed in recent studies on the albino rat, in which it was observed that both REM-sleep loss and the subsequent REM-sleep rebound were quantitatively related to the thermal load (duration of the exposure × decrease in ambient temperature with respect to normal laboratory conditions).17-19 It has also been shown that the amount of NREM sleep is less affected during recovery, since no substantial increase in its amount has been found in either the cat1,2 or the rat.14 However, an increase in the electroencephalogram (EEG) power density in the delta band (0.75-4 Hz) during NREM sleep has been observed in the rat.14 Cold Exposure and Sleep in the Rat: Effects on Sleep Architecture and the Electroencephalogram