 Other fish seem to sleep, however.For example, zebrafish, tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night (or by day, in the case of the swell shark); Spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response.A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep.