A pilot study on the relationship between sleep restriction, endogenous testosterone and cognitive performance

Aims: Both high endogenous testosterone and sleep restriction impact adversely on cognitive performance. The normal daily testosterone rhythm may also be disrupted by sleep restriction. As such, sleep restriction could degrade cognitive performance both directly and through its effect on testosterone. This study investigated the relationship between sleep restriction, testosterone levels and cognitive functioning. Methods: Fourteen healthy men (mean ± SD, age: 27.4 ± 3.8 yr; BMI: 23.5 ± 2.9 kg/m2) with normal plasma glucose and lipid levels and normal liver, renal and thyroid function participated in a live-­‐in, laboratory-­‐based sleep restriction protocol. Participants underwent two baseline nights (B1, B2; 10 h time in bed (TIB); 22:00 h -­‐ 08:00 h) followed by five nights of sleep restriction (SR1 5; 4 h TIB; 04:00 h -­‐ 08:00 h) and one recovery night (10 h TIB; 22:00 h -­‐ 08:00 h). The timing of meals and caloric intake were strictly controlled, and only non-­‐vigorous physical activity was allowed. Blood was sampled on B1 and SR5 via an indwelling catheter at 09:00 h, and then every 2 h from 10:00 h until 20:00 h. Lapses in sustained attention were assessed using the Psychomotor Vigilance Test (PVT) and subjective sleepiness was assessed using the Karolinska Sleepiness Scale (KSS) throughout scheduled waking periods. On B1 and SR5, the PVT and KSS were completed at 11:00 h, 12:30 h, 16:30 h and 19:30 h. The relationships between sleep restriction, endogenous testosterone and cognitive performance were assessed using mixed effects linear regression comprising effects of day, trial and day*trial interaction. Testosterone was entered as a covariate, and a day*testosterone interaction effect was examined to determine whether there was a modulatory influence of testosterone on PVT lapses and KSS subjective sleepiness. Results: There was a significant effect of sleep restriction on PVT lapses (F(1,83) = 41.4, p < .001) and KSS sleepiness (F(1,83) = 51.9, p < .001) but not on testosterone (F(1,90) = 4.1, p = .08). There was no main effect of testosterone on PVT lapses (F(1,82) = 0.92, p = .34) or subjective sleepiness (F(1,82) = 0.04, p = .85). However, higher endogenous testosterone levels were associated with greater changes after sleep restriction in PVT lapses (F(2,81) = 12.3, p < .001) and subjective sleepiness (F(2,81) = 12.7, p < .001). Conclusions: The findings suggest that young adult men with higher endogenous testosterone levels (within the normal range) experience greater degradation of cognitive functioning following 5 days of sleep restriction. This relationship may point to endogenous testosterone as a modulator of individual vulnerability to sleep restriction. Further investigation is needed to determine the mechanisms underpinning this relationship.