Monotremes and the evolution of rapid eye movement sleep.
Level 5 - mechanism / opinion, no new human data
Animal research and narrative review (Level 5 by design analogy)
PubMed 9720111 · doi:10.1098/rstb.1998.0272
What was done
The authors summarized comparative neurophysiological and behavioral sleep studies in two monotreme species: the short-beaked echidna and the platypus. In the echidna, neuronal unit activity was recorded from mesopontine brainstem regions alongside cortical electroencephalogram (EEG) monitoring. In the platypus, cortical EEG was recorded during natural sleep and waking states alongside behavioral observation of rapid eye, bill, and head twitching.
What was found
The abstract reports no quantitative numerical values. In echidnas, brainstem units during sleep with high-voltage cortical EEG fired in irregular bursts with an intensity intermediate between regular non-REM and highly irregular REM patterns seen in placental mammals. In platypuses, sleep was accompanied by vigorous rapid eye, bill, and head twitching characteristic of REM sleep, occurring alongside moderate- to high-voltage cortical EEGs. The platypus was reported to have more REM sleep than any other animal.
Why it matters
These findings challenge the classical hypothesis that REM sleep evolved late in mammalian phylogeny after the divergence of monotremes. Instead, the presence of brainstem and behavioral REM features in monotremes suggests that REM sleep, or a precursor state, evolved earlier in mammalian or ancestral reptilian lineages.
Limits
The abstract is a narrative review summarizing primary experiments and does not report sample sizes, quantitative sleep architecture durations, statistical comparisons, or variability metrics. Behavioral and electrophysiological observations were restricted to two non-human species under laboratory recording conditions.
Cited by
- supports Echidnas experience REM sleep, contrary to earlier scientific belief.