Sleepiness, sleep duration, and human social activity: An investigation into bidirectionality using longitudinal time-use data

Abstract

Daytime sleepiness impairs cognitive ability, but recent evidence suggests it is also an important driver of human motivation and behavior. We aimed to investigate the relationship between sleepiness and a behavior strongly associated with better health: social activity. We additionally aimed to investigate whether a key driver of sleepiness, sleep duration, had a similar relationship with social activity. For these questions, we considered bidirectionality, time of day, and differences between workdays and days off. Over 3 wk, 641 working adults logged their behavior every 30 min, completed a sleepiness scale every 3 h, and filled a sleep diary every morning (rendering >292,000 activity and >70,000 sleepiness datapoints). Using generalized additive mixed-effect models, we analyzed potential nonlinear relationships between sleepiness/sleep duration and social activity. Greater sleepiness predicted a substantial decrease in the probability of social activity (odds ratio 95% CI = 0.34 to 0.35 for days off), as well as a decreased duration of such activity when it did occur. These associations appear especially robust on days off and in the evenings. Social duration moderated the typical time-of-day pattern of sleepiness, with, for example, extended evening socializing associated with lower sleepiness. Sleep duration did not robustly predict next-day social activity. However, extensive social activity (>5 h) predicted up to 30 min shorter subsequent sleep duration. These results indicate that sleepiness is a strong predictor of voluntary decreases in social contact. It is possible that bouts of sleepiness lead to social withdrawal and loneliness, both risk factors for mental and physical ill health.

Keywords: interpersonal relations; sleep; sleepiness; social behavior; time-use.

Fig. 1.

Stacked proportions of average daily behavioral activity as reported by participants in the time-use survey.

Fig. 2.

Probability of being socially active for three intraindividual sleepiness levels at different times of day, split by type of day. High sleepiness represents a KSS of two steps above the participant's own average. Low sleepiness represents a KSS of two steps below the participant's own average. Average sleepiness is the mean sleepiness for each participant. Error ribbons represent pointwise 95% CIs. Since predictions are made independently of participants, each line does not necessarily represent a day when any specific person is sleepy/average/alert but rather the probability of any given person being socially active at that time point and sleepiness level.

Fig. 3.

Predicted duration of social activity (per 3-h chunk) for individuals who engaged in social activity at least once. The duration is shown for three sleepiness levels at different times of day, split by type of day. Low sleepiness represents a KSS of two steps below the participant's own average. Average sleepiness is the mean sleepiness for each participant. High sleepiness represents a KSS of two steps above the participant’s own average. To increase ease of interpretability, predicted 30-min periods of social activity have been converted to predicted minutes of social activity. Error ribbons represent pointwise 95% CIs. Since predictions are made independent of participants, each line does not necessarily represent a day when any specific person is sleepy/average/alert but rather a prediction of the average length of social activity for a given 3-h time chunk and sleepiness level.

Fig. 4.

Predicted duration of social activity (per day) for individuals engaged in social activity at least once. Social activity duration is shown to be dependent on intraindividual sleep duration and how much sleep one obtained on average during the study, split by type of day. In this figure, we exemplify the predicted effect of a change in interindividual sleep duration for three example sleepers. Note that each line does not represent any individual participant, but rather the predictions made by the model for a particular length of average sleeper as arbitrarily defined as follows. An average sleeper was defined as an individual whose average was equal to the mean of all participants (473 min, i.e., 7 h 53 min). A short sleeper was defined as an individual whose average sleep duration was two SDs (39 min) below the mean of all participants’ mean sleep durations. Therefore, for this model, a short sleeper is someone who averages 394 min (6 h, 34 min) or less of sleep per night. A long sleeper was defined as an individual whose sleep duration was two SDs above the sample mean, in other words, someone who averaged 551 min (9 h, 11 min) or more of sleep per night. To increase ease of interpretability, the predicted sum of daily 30-min periods of social activity have been converted to hours. Error ribbons represent pointwise 95% CIs.

Fig. 5.

Plot representing the association between duration of social activity during the day and within-subject centered sleep duration (i.e., representing deviation from individual mean) during the following night. To increase ease of interpretability, 30-min periods of social activity have been converted to hours of social activity.