Changes in sleep in response to 6 and 12 h of total sleep deprivation (TSD). Each data point represents the average ± SEM cumulative change in total sleep time, non-REM sleep, and REM sleep during 6 h (left) and 12 h (right) of TSD and the recovery sleep period. To calculate the cumulative change, hourly amounts of non-REM and REM sleep on the baseline day were subtracted from those on the sleep deprivation day, resulting in a cumulative deficit by the 6th and 12th hours of sleep deprivation. The cumulative deficit was then tallied over the recovery period. There were no significant differences in the rate of the sleep deficit, and both groups recovered from the deficit at the same rate.