Twenty-four healthy, young (21-35 years old) men with no complaints of daytime sleepiness, no habitual napping, and polysomnographically verified normal nocturnal sleep extended their time in bed (TIB) to 10 h for 6 consecutive nights to assess the effects of sleep extension on daytime sleepiness and performance. Twelve subjects had basal average daily sleep latencies of less than or equal to 6 min on the Multiple Sleep Latency Test and 12 had latencies of greater than or equal to 16 min before TIB was extended. The sleep extension improved daytime sleepiness differentially in the two groups. The degree of improvement was greater in the sleepy subjects than the alert subjects and the pattern of improvement differed between the groups. Sleepy subjects showed an immediate and uniform increase in alertness, while alert subjects did not show improvements until late in the extension. However, sleepy subjects never achieved the baseline level of sleepiness/alertness seen in the alert subjects.

The alerting effects of caffeine were assessed using a standard physiological measure of daytime sleepiness/alertness, the Multiple Sleep Latency Test (MSLT). Healthy young men (n = 24) were randomly assigned to receive caffeine 250 mg or placebo administered double blind, at 0900 and 1300 hours on each of 2 days. On the 3rd day both groups received placebo to test for conditioning to the alerting effects of caffeine. Each day sleep latency was measured at 1000, 1200, 1400, and 1600 hours and performance (divided attention at 1030 hours and auditory vigilance at 1430 hours) was assessed. Caffeine increased sleep latency (i.e., improved alertness) and auditory vigilance performance compared to placebo. Tolerance to the effects of caffeine on sleep latency developed over the four administrations. On the conditioning test (day 3) the group receiving caffeine the previous two days was more alert and performed better than the placebo group.

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Ethanol (0.5 g/kg) was administered to 12 healthy, normal-sleeping men, aged 21 to 45, at two different times of the day (0900 and 1700 hr). The Multiple Sleep Latency Test (MSLT) was conducted at 1000, 1200, 1400, and 1600 hr in the day drinking condition and at 1800, 2000, 2200, and 2400 hr in the evening drinking condition. On placebo, sleepiness was greater in the daytime testing hours than in the evening, replicating findings on the circadian rhythm of sleepiness/alertness. There was a time of drinking (day versus evening) by ethanol interaction. An ethanol effect on sleep latency was found in the daytime hours, when alertness was relatively low. Ethanol failed to have a significant effect on sleep latency during the evening hours when alertness levels were increasing. Performance on a divided attention task, administered 1 hr postconsumption, was impaired by ethanol consumption, but did not vary as a function of time of drinking (day versus evening). However, at 5 hr postconsumption, mean reaction time on the first 20 min of a 40-min auditory vigilance task was slowed by ethanol to a greater extent after day drinking then after evening drinking.

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