Do Other Mammals Dream?

First, no one is sure why we, or any other animal, dream. During the past century, however, scientists have made significant progress in understanding the factors that influence sleep and dreaming. Evidently, our nights are influenced by what we’re doing during the day. We spend our days gathering information—some of it we would like to keep, but most of it we need to discard. If we do not clear out our mental storage space regularly we risk saturating our brain with too much useless trivia. To fully process this information, we need to shut our brain down—disconnect it entirely from the outside world. The challenge is that we only have one brain to work with. How do mammals solve this problem?

About 2 million years ago (that’s the current guess) brains became sufficiently complex that they were able to solve the problem of memory processing by dividing the night into two types of sleep: rapid eye-movement (REM) sleep and slow wave or non-REM sleep. [See Wenk, 2017, for a more detailed discussion] We can, and do, dream in either phase of sleep but the characteristics of our dreams differ greatly. It appears that the major difference in terms of the content of REM dreams versus non-REM dreams is that REM dreams involve more emotional, visual, and movement content, are longer, show more continuity and coherence, and are more vivid than non-REM dreams. During non-REM sleep, our dreams are often more static, involve older memories, or are quite emotionally charged. Our nightmares usually occur during non-REM sleep.

The presence of dreams during REM sleep, and the detection of physiologically similar states in other mammals, suggests that other animals experience dreaming. We can never know for certain if other species have the same mental experiences as humans, but to the extent that their brain electrophysiology and behavior overlaps with ours, we can infer certain parallels.

The most ancient of mammals, the monotremes, which include the platypuses, echidnas, and egg-laying mammals, have unique departures from the typical EEG states of other sleeping mammals. Like humans, they have two distinct sleep states defined as non-REM and REM sleep state (with the familiar characteristics of humans that include muscle atonia, rapid eye movements, twitching, and erratic heart rates). Although they experience lots of REM sleep, it is unusual. Unlike in placental mammals, where the EEG is desynchronized with a low-voltage activity, monotreme REM sleep shows synchronized high-voltage activity, which is more like human non-REM sleep. This unique pattern of EEG suggests that monotremes, if they dream, probably experience less vivid, shorter, and disconnected dreams associated with human non-REM sleep.

Cetacean sleep — whales, dolphins, and porpoises — is most conspicuously characterized by uni-hemispheric slow wave sleep. One hemisphere shows desynchronized low-voltage activity characteristic of being awake, while the other hemisphere shows synchronized high-voltage activity characteristic of non-REM sleep. In addition, since they need to swim to breathe, cetaceans are moving while sleeping and they always keep one eye open — the one connected to the hemisphere showing non-REM. Cetaceans are also notable for their lack of sleep during the first few months of life. Their ability to have only one hemisphere asleep is due to the highly altered anatomy of their brain systems that control sleep. The fact that no one has discovered any form of REM sleep in these mammals has led to the conclusion that vivid REM sleep dreams are likely to be completely absent in the cetaceans.

Life in the water has a unique impact on sleeping and dreaming for other mammals. It appears that when sleeping on land, seals have the potential to experience both non-REM and REM-type dreams like other mammals. In contrast, when they are in the water, the potential for REM-type dreams is absent. The potential for non-REM-type dreams may be more like that observed in cetaceans rather than land-based mammals. While both non-REM and REM sleep have been observed in manatees, their slow-wave activity is like that in cetaceans, probably related to their aquatic lifestyle. However, manatees experience total whole-brain REM sleep for only about 15 minutes per day. It is likely that both non-REM and REM type dreams could be experienced by manatees although their brain anatomy and chemistry suggest that the content and intensity of the non-REM dreams in manatees may not be as rich as potentially experienced in other mammals.

The African elephant is the shortest sleeping mammal recorded to date, having only two hours of sleep on average each day. Furthermore, they mostly sleep standing up, which suggests that they primarily experience non-REM sleep. This assumption is based upon the knowledge that muscle atonia is indicative of REM sleep in other mammals. Elephants may only experience REM sleep on those few occasions when they sleep while lying down. Studies indicate that the neural systems responsible for sleep patterns in other mammals are present in elephants, and do not differ significantly from other mammals, suggesting that if elephants do dream, they probably experience both the non-REM and REM types of dreams.

Evidence that dreaming occurs during both non-REM and REM sleep indicates that all mammals have the potential to experience dreams of some nature.