Background and purpose: This study compared the effects of caffeine in patients with primary insomnia and normal volunteers. The main goal was to determine the differences in sensitivity to caffeine between the groups. We investigated the effects on daytime sleep of placebo or caffeine after a night of total sleep deprivation (SD). We hypothesized that insomniacs would be more affected by caffeine, which would suggest a change in adenosine receptor (number or sensitivity) in primary insomniacs.
Patients and methods: Six primary insomnia patients (Diagnostic and Statistical Manual of Mental Disorders (DSM-IV)) and six normal volunteers with no sleep complaints participated in a double-blind study with caffeine or placebo administered under a cross-over design with each subject serving as his or her own control. The participants did not have a history of drinking coffee or caffeinated beverages. Data from all-night polysomnography and multiple sleep latency test (MSLT) were collected in the sleep research laboratory of National Institute of Medical Sciences and Nutrition Salvador Zubirán.
Results: During the baseline night, patients with insomnia had significantly less delta sleep and less total sleep time than the normal volunteers. Mean sleep latency under basal MSLT did not differ between the groups. However, insomnia patients had significantly less total sleep during each nap compared to normal volunteers. After one night of total SD and under caffeine administration, the insomniacs had significantly longer sleep latency and less total sleep time in MSLT compared to normal volunteers. After SD, healthy volunteers reduced sleep latencies in MSLT with or without caffeine.
Conclusions: Patients with insomnia had a higher sensitivity to the diurnal awakening effect of caffeine even after one night of SD. This suggests that changes in the adenosine receptors could, in part, be responsible for the hyperarousal state that has been reported in primary insomnia.