Method for EEG measurement in a human sleeping in a highly concentrated Epsom salt solution (float tank)

Purpose: Dolphins sleep with only one half of their brain at a time. This phenomenon is called unihemispheric slow-wave sleep. The northern fur seal has been reported to do this whilst in the water but to sleep bihemispherically when terrestrial. These observations made us wonder whether this ability might be evolutionarily conserved in humans if we simply slept in water. The methodological problem was how to record an electroencephalogram (EEG) when somebody is floating. Methods: We built a waterproof and temperature-regulated housing to protect an EEG recorder (Alice PDx; Philips Respironics, Pittsburgh, PA, USA) from being near a restricted environmental stimulation therapy floatation tank (aka floatation reduced environmental stimulus therapy [REST] therapy) with very-low-level light and noise and thermoneutral temperature. Float therapy uses a highly concentrated Epsom salt solution, thus making the human body extremely buoyant. We recorded the EEG at Fp1 and Fp2 and also eye movements via an electrooculogram (EOG) using surface electrodes. We used the American Academy of Sleep Medicine guidelines to score sleep (v2.2). A single male human was recorded in the float tank during two nap opportunities. Results: The recorded EEG detected unequivocal stage 2 sleep characteristics during both nap opportunities. At no point did we detect unihemispheric slow-wave sleep. Conclusion: Humans can fall asleep in water. We recorded the EEG and EOG and saw clear evidence of unequivocal sleep. We saw no asymmetry in the sleep but have not yet captured an extended period of slow waves to see whether humans sleep like dolphins when they sleep in water.