The influences of time-of-day and sleep deprivation on postural control.

The aim of this study was to check the combined and/or dissociated influences of time-of-day and sleep deprivation on postural control. Twenty subjects participated in test sessions which took place at 6:00 am, 10:00 am, 2:00 pm and 6:00 pm either after a normal night's sleep or after a night of total sleep deprivation. Postural control was evaluated by COP surface area, LFS ratio and Romberg's index. The results showed that postural control fluctuates diurnally according to three different periods, pronounced by sleep deprivation: (1) at 6:00 am, there was no modification by sleep deprivation; (2) at 10:00 am and 2:00 pm, an interaction effect was observed for COP surface area and LFS ratio after sleep deprivation. Values of COP surface area were significantly higher (P < 0.01) following the night of sleep deprivation than after the normal night's sleep (139.36 ± 63.82 mm² vs. 221.72 ± 137.13 mm² and 143.78 ± 75.31 mm² vs. 228.65 ± 125.09 mm², respectively); (3) at 6:00 pm, the LFS ratio was higher than during the two other periods (P < 0.001) whereas COP surface area decreased to the level observed at 6:00 am. At this time-of-day, only the LFS ratio was significantly increased (P < 0.05) by the night of sleep deprivation (0.89 ± 0.14 vs. 1.03 ± 0.30). This temporal evolution in postural control does not seem to be related to any deterioration in visual input as Romberg's index (150.09 ± 97.91) was not modified, regardless of the test session.