Introduction: The effects of microgravity during spaceflight, or with prolonged bed rest, on the brain have received attention in relation to a visual impairment and intracranial pressure syndrome in astronauts returning from the International Space Station. However, the associated changes in neuroanatomy, changes in cognition and neuroelectrical activity and connectivity during wake and sleep have not been studied extensively. In addition, a significantly shortened and disrupted sleep has also been consistently reported in space missions. The aim of the present study was to determine the effects of seven days of supine unloading on a hyper-saline filled water bed (hyper buoyancy floatation, HBF), an Earth-based novel analogue of microgravity, on slow wave activity (SWA) power (1–4 Herz, Hz) topography.Methods: Twelve healthy male subjects, aged (27.2±4.2 years), with no previous neuro/psychiatric history, underwent overnight in-laboratory polysomnography (PSG/EEG) recordings during seven days of an unloading period, collected with CPz referencing (64-EEG-channels; Ant Neuro, Germany). Sleep staging was performed according to standard criteria (S.H.). For the duration of the intervention period, the subjects lied supine, followed controlled sleep/wake schedules as per our laboratory protocol, they were fed a controlled diet and allowed a maximum of 15 minutes per day off the HBF (for personal hygiene etc). Results: The unloading period of seven days resulted in a significant (p<0.05) change in SWA over centro-frontal regions relative to the rest of the brain compared. Global EEG power spectra (power spectral density averaged across all scalp channels) did not differ between the first and last night recording, for any of the sleep stages (N1 1 , N2, N3, REM sleep, Wake). The significant decrease in SWA power spanning the frequency range 0.5-2.5 Hz was most prominent during N3 and REM. However, during REM, a wider fronto-occipital region was affected. Conclusion: Overall, results suggest the presence of local SWA power differences induced by intervention period of only seven days on HBF, a novel Earth-based analogue of microgravity. Further studies are required to investigate potential mechanisms underlying the observed SWA changes and their possible role in previously reported cognitive changes under microgravity conditions.