Changes in postural sway and cortical activities after napping.

Lowered arousal state after napping may lead to poorer standing balance and the need to recalibrate the sensory organization system. This study aimed to examine the changes in postural sway and sensory associated cortical activities immediately after waking from a nap. A convenience sample of young adults (7 males and 5 females, 21.0 ± 2.3 yr.) was recruited. Before and after a 50-min lying-down nap, participants were asked to stand quietly with eyes open/closed on a firm/foam surface, and electroencephalography (EEG) in theta, alpha, beta, and gamma bands in sensory association areas was recorded. All participants self-reported that they fell asleep during the 50-min period provisioned for nap (Karolinska Napiness Scale before nap 4.2 ± 1.1, after nap 5.7 ± 0.8). The average time taken to finish data collection after waking the participants was 19.0 ± 4.0 minutes. The results showed less postural sway (t11 =  2.726, p =  0.02) and increased frequency of postural sway (t11 =  -3.339, p =  0.007) after nap in the eyes-open firm-surface condition. The EEG results revealed decreased activity in the alpha (F1,9 =  15.540, p =  0.003) and gamma (F1,9 =  6.626, p =  0.030) bands in the right parietal area after nap, and increased beta power in the left occipital area (Z =  -2.241, p =  0.025). In conclusion, after waking from a nap, healthy adults show increased changes in direction of postural sway which is effective in decreasing postural sway in eyes-open firm-surface condition. Even in healthy adults without worsen postural performance after nap, the EEG results suggested a decrease of efficacy in dealing with sensory challenges within twenty minutes post napping. This study contributes to the understanding of the mechanisms underlying changes in balance control after napping, which might help fall prevention programs for the elderly.