Microgravity reduces sleep-disordered breathing in humans

To understand the factors that alter sleep quality in space, we studied the effect of spaceflight on sleep-disordered breathing. We analyzed 77 8-h, f ull polysomnographic recordings (PSGs) from five healthy subjects before sp aceflight, on four occasions per subject during either a 16- or 9-d space s huttle mission and shortly after return to earth. Microgravity was associat ed with a 55% reduction in the apnea-hypopnea index (AHI), which decreased from a preflight value of 8.3 +/- 1.6 to 3.4 +/- 0.8 events/h inflight. Thi s reduction in AHI was accompanied by a virtual elimination of snoring, whi ch fell from 16.5 +/- 3.0% of total sleep time preflight to 0.7 +/- 0.5% in flight. Electroencephalogram (EEG) arousals also decreased in microgravity (by 19%), and this decrease was almost entirely a consequence of the reduct ion in respiratory-related arousals, which fell from 5.5 +/- 1.2 arousals/h preflight to 1.8 +/- 0.6 inflight. Postflight there was a return to near o r slightly above preflight levels in these variables. We conclude that slee p quality during spaceflight is not degraded by sleep-disordered breathing. This is the first direct demonstration that gravity plays a dominant role in the generation of apneas, hypopneas, and snoring in healthy subjects.