The acute effects of sleep restriction therapy for insomnia on circadian timing and vigilance

Sleep restriction therapy (SRT) has been shown to improve insomnia symptoms by restricting sleep opportunity. Curtailment of time in bed affects the duration and consolidation of sleep, but also its timing. While recent work suggests that people with insomnia are characterised by misalignment between circadian and behavioural timing of sleep, no study has investigated if SRT modifies this relationship. The primary aim of this study was to examine change in phase angle after two weeks of SRT. As a secondary aim, we also sought to assess the effect of SRT on psychomotor vigilance. Following a one-week baseline phase, participants implemented SRT for two consecutive weeks. Phase angle was derived from the difference between the decimal clock time of dim light melatonin onset (DLMO) and attempted sleep time. Secondary outcomes included vigilance (assessed via hourly measurement during the DLMO laboratory protocol), sleep continuity (assessed via sleep diary and actigraphy), and insomnia severity. Eighteen participants meeting insomnia criteria (mean age=37.06+8.99) took part in the study. In line with previous research, participants showed robust improvements in subjective and objective sleep continuity, as well as reductions in insomnia severity. The primary outcome (phase angle) was measurable in 15 participants and revealed an increase of 34.8min (~0.58hrs; 95%CI=0.01-1.15) from baseline to post-treatment (2.27+0.94hrs vs 2.85+1.25hrs). DLMO remained relatively stable (20:49hr vs 21:01hr), while attempted sleep was 46.8min later (~0.78hrs; 95%CI=0.41-1.15; 23:05hr vs 23:52hr). With respect to psychomotor vigilance, reaction time was delayed (by 52.71ms, 95%CI=34.44-70.97) and number of lapses increased (by 5.84, 95% CI=3.93-7.75) following SRT. We show that SRT increases phase angle during treatment, principally by delaying the timing of sleep attempt. Future studies are needed to test if an increase in phase angle is linked to clinical improvement. Finally, reduction in vigilance following SRT appears to be of similar magnitude to normal sleepers undergoing experimental sleep restriction, reinforcing the importance of appropriate safety advice during implementation.