RESULTS	Comparison of sleep EEG power density spectra of frontal ( F3A2 ) and central ( C3A2 ) derivations at altitudes compared to baseline revealed that slow-wave activity ( SWA , 0.8-4 .6 Hz ) in non-REM sleep was reduced in an altitude-dependent manner ( ~ 4 % at 1630 m and 15 % at 2590 m ) , while theta activity ( 4.6-8 Hz ) was reduced only at the highest altitude ( 10 % at 2590 m ) .\