Huge ones like the dolphin are reported to sleep for a little over 10 hours daily. Compare this with a bonnet monkey whose REM sleep is only 12% and the remaining 88% is non-rapid eye movement (NREM) sleep. The percentage of REM sleep across species is roughly correlated with cognition, brain development and activity. The REM sleep is highest in growing infants (often up to 50%), indicating that their brains are busy collecting, collating, consolidating and using information. (Indeed, a similar growth-dependent behaviour in the REM sleep patterns in chimpanzees has been noted by Japanese primatologists).

Source: The Hindu February 05, 2017 12:25 UTC