In Chapter 1 (Biological rhythms and sleep) of our A2 Complete Companion there is a contradiction (kindly pointed out by Ruth Bailey of Akeley Wood School). On page 13 the text says that dolphins don’t have REM sleep whereas on page 14 the graph indicates a significant amount of REM sleep in dolphin. So which is correct?
The data for the graph was taken from a study Lyamin et al. (2004) of one dolphin, reported by the Phylogeny of Sleep Project (you can see the dolphin data here). As pointed out in our textbook much of the data about sleep is actually derived from very small samples and research conducted under poorly controlled conditions.
All of the other dolphin studies given by the Phylogeny of Sleep Project did not record the amount of REM sleep which is why we used the data from Lyamin et al. However this data is misleading as the general view appears to be that (REM) sleep is either absent in cetaceans (e.g. dolphins) or occupies an extremely small proportion of the day – an absolute maximum of 15 min each day (Manger et al., 2003). In fact a recent paper published by Lyamin et al. (2008) states that ‘We find that for cetaceans sleep is characterized by USWS [unihemispheric slow wave sleep] [and] a negligible amount or complete absence of rapid eye movement (REM) sleep’.