Sleep spindles and arousal: the effects of age and sensory stimulation

This study assessed a proposed sleep-preserving role for sleep spindles by evaluating variations in this activity as a function of factors, both naturally occurring and experimentally induced, known to affect and effect arousal from sleep. These factors included age, auditory stimulation, and experimentally induced arousal from sleep. Analyses were based on data from 84 males (5-49 yrs. old) from normal and clinical (hyperactive, enuretic, and chronic pain) populations who had participated in sleep auditory arousal threshold studies involving adaptation and 1-2 experimental nights. Spindles on experimental nights were visually analyzed and incidence determined for the two minutes preceding and throughout all Stage 2 arousal attempts. Prestimulation spindle occurrence in 39 preadolescent subjects with two experimental nights did not vary significantly from night-to-night, and prestimulation period comparisons between clinical groups and their respective controls were also non-significant. Anticipated relationships between spindle activity and indices of arousal-either inverse with respect to known variations in arousal threshold, i.e., decreases with age and across the night, or direct with respect to stimulus intensity particularly on trials when arousal did not occur-were not observed. Instead, all age groups showed significant decreases in spindle density as an increasing negative function of stimulus intensity. These findings suggest that to the extent to which sleep spindles can be considered to play a role in sleep preservation by inhibiting or attenuating potentially arousing stimuli, these effects appear to be restricted to endogenously generated stimuli and are passive rather than reactive in nature.