6.7 Sleep, the brain and hibernation

There has been a popular misconception that hibernating animals are asleep when dormant, and that arousal during or at the end of hibernation involves waking analogous to that following deep sleep. Sleep in homeothermic animals can be divided into several phases, each with distinct patterns of electrical activity in the brain, as measured by an electroencephalogram (EEG). The passage into sleep is a transition from wakefulness into the stage called slow-wave sleep (SWS). SWS, and its characteristic electrical pattern of brain waves in the frequency range 0.7–4.0 Hz, is interrupted by periods of sleep characterized by, among other things, rapid eye movements, loss of muscle tone in the head and neck, and loss of a shivering response. These periods are known as rapid eye movement (REM) sleep. Dreaming occurs mainly at this time, and blood flow to the brain is markedly increased. The electrical patterns shown in REM sleep are very close to those of wakefulness.

Sleep deprivation and hibernation in Djungarian hamsters both lead to the suppression of slow-wave sleep activity (SWA); probably caused in the latter case by the drop in core body temperature, and a burst of SWA occurs on arousal from hibernation, further supporting the idea that the animal shows the signs of lack of sleep rather than the opposite. Stronger evidence for this theory comes from research showing that hibernation in Richardson's ground squirrel (Spermophilus richardsonii) leads to an accumulation of nearly three times the basal level of oleamide, a derivative of the fatty acid oleic acid, which builds up in the brain during sleep deprivation.

SWA normally follows changes in sleep activity and core body temperature during the REM sleep of euthermic animals, but still exhibits a daily cycle during non-REM sleep. In animals that display bouts of torpor, daily temperature fluctuations continue but sleep and SWA cycles shorten to 25% of their normal length. This change may be required to maintain torpor bouts lasting several days that are typically observed in hibernating mammals. Circadian rhythms are maintained by the SON of the hypothalamus, and thus we would expect that the modification of sleep patterns in hibernators is under neural control from the same brain centre as that which determines cycles in other body functions.