Study
Do all animals sleep?
Siegel, Jerome M.
Trends in Neurosciences, 2008
A review of what is actually established about sleep across animal groups, setting the behavioural criteria — reduced responsiveness, species-typical posture, rapid reversibility and homeostatic rebound after deprivation — against the physiological criteria that require electrophysiology, and examining which animals have been tested against which.
- Studied in
- Mammalia, Aves, Insecta, Actinopterygii
- Sample size
- review across vertebrate and invertebrate groups
Behavioural sleep is documented very widely, including in insects and molluscs. Electrophysiological sleep of the mammalian kind, with distinct non-REM and REM states, is confirmed in mammals and birds and is either absent or unrecognisable in most other groups. Sleep duration varies enormously between species and correlates poorly with brain size, longevity or the popular assumption that intelligence requires more of it.
What it means
The authors' reading, and ours. Where they differ, that difference is the point.
How the authors put it
Sleep is best treated as a family of regulated inactive states rather than one phenomenon, and claims that every animal sleeps depend heavily on which definition is being used.
How NatureHQ reads it
This is the paper that makes the whole subject writable honestly. "Do animals sleep?" has no single answer because "sleep" has at least two definitions, and the popular version silently uses the mammalian one everywhere. The rebound criterion is the useful discipline: an animal that is merely still has shown nothing, and an animal that sleeps longer after being kept awake has shown that something is being regulated.
- Coverage is very uneven — a handful of laboratory species dominate the physiological data.
- Absence of recorded sleep in a group often means absence of recording rather than absence of sleep.
- Deprivation experiments are stressful, and stress has effects of its own that are hard to separate.
What this study is used for on NatureHQ
One study can inform several subjects. Here is everywhere this one is cited.
Published by: Elsevier journalsPublished in an Elsevier journal.
doi.org/10.1016/j.tins.2008.02.001
This reference was resolved automatically against Crossref on 2026-08-11.
NatureHQ summarises research in its own words and does not reproduce published text. Reviewed 2026-08-10.