The influence of sleep deprivation during the first part of the night on 24-h ambulatory blood pressure monitoring (ABPM) was studied in 18 normotensive subjects. They underwent two ABPM, one week apart: during the first, they slept from 11 PM to 7 AM, and during the second, from 2 AM to 7 AM. The main differences were observed at dawn, before awakening, when SBP and DBP significantly decreased (P < .01) in the restricted sleep regimen, and during the morning after the recovery sleep, when SBP and HR significantly increased (P < .05). The explanation for these findings is not obvious. We suppose that the decrease in SBP and DBP at dawn might be due to a reorganization of the sleep phases in the restricted sleep regimen, whereas the increase in SBP and HR after awakening might be due to a greater sympathetic activation, as though sleep deprivation was a stressful condition.