Interindividual variation in sleep duration and its association with sleep debt in young adults

Abstract

Study objective: To determine whether variation in sleep duration reflects variation in sleep need or self-imposed sleep restriction.

Design: After habitual bedrest duration of participants was assessed during a 2-week outpatient protocol, volunteers were scheduled to sleep according to this schedule for 1 week prior to and the first night after admission to a general clinical research center. The inpatient protocol included multiple sleep latency testing on the second day and sleep recordings during a bedrest extension protocol that included 16 hours of sleep opportunity (12 hours at night and 4 hours at midday) for 3 consecutive days.

Setting: Outpatient monitoring followed by inpatient assessment of sleep.

Participants: Seventeen healthy volunteers (10 women) aged 18-32 years without clinical sleep disorders.

Interventions: Extension of sleep opportunity.

Measurements and results: The habitual bedrest duration varied from 6.1 to 10.3 hours. Individuals with shorter habitual bedrest duration fell asleep more quickly and frequently during the multiple sleep latency test than did those with longer habitual bedrest duration. On the first day of extended sleep opportunity, the total sleep time of all individuals was greater than their habitual bedrest duration; the average increase in total sleep time was 4.9 hours (P = 0.001). The increase in total sleep time declined across the 3 day bedrest-extension protocol (P = 0.003 for trend). During the third day of increased sleep opportunity, the total sleep time was negatively associated with habitual bedrest duration (P = 0.005); individuals with shorter habitual bedrest duration continued to sleep more than those with longer habitual bedrest duration.

Conclusion: Those individuals with shorter habitual sleep durations carry a higher sleep debt than do those with longer habitual sleep duration. Interindividual variation in sleep duration may primarily reflect variation in self-selected sleep restriction or wake extension.

Figure 1

First inpatient sleep episode. Association between habitual bedtime duration (HBD) and sleep parameters during the first inpatient sleep episode (for which time in bed was equal to HBD). Each point represents a different volunteer. Regression lines are shown if the slope is significantly different from 0. NREM refers to non-rapid eye movement; TST, total sleep time; SWS, slow-wave sleep; REM, rapid eye movement.

Figure 2

Habitual bedtime duration and Multiple Sleep Latency Test results. Association between habitual bedtime duration (HBD) and daytime sleep propensity as assessed by the Multiple Sleep Latency Test (MSLT). For each volunteer, the sleep latency for each of the 5 sleep opportunities (circles) and median (horizontal line) during the MSLT are shown. If the latency was the same on 2 or more of the sleep opportunities, only 1 circle is plotted.

Figure 3

Sleep during first the first inpatient bedrest episode and the following three days of bedrest extension relative to habitual bedtime duration. The solid lines indicate regression lines for which the slope was statistically significant. The dotted lines for total sleep time (TST) (top 4 panels) are the lines at which TST equals habitual bedtime duration (HBD), which is what would be expected if HBD reflected steady-state TST at 100% sleep efficiency. The left column contains data for TST, slow-wave sleep (SWS), and rapid eye movement (REM) sleep during the first inpatient bedrest episode (time in bed equal to HBD for each volunteer); these data are repeated from Figure 1 for comparison of changes across the days of the protocol. The other 3 columns contain data from the 3 days of bedrest extension during which total time in bed was 16 hours for all volunteers.