Introduction: The social effects of insufficient sleep represent a relatively new area of study in sleep research. For instance, the influence of sleep on verbal communication, particularly communicative perspective-taking and adaptation based on audience, is underexplored. Furthermore, studies on how sleep affects speaking patterns are limited, although slowed speech has been indicated as an effect of sleep loss. Given the importance of effective communication in all areas of life, understanding the harmful effects of insufficient sleep on communication is vital. The present study investigated whether sleep restriction affects speech speed and the ability to adjust one's speech depending on the listener.

Method: In an experimental cross-over study, 273 participants described nine abstract figures separately to a child and an adult, both of whom were depicted as photos on a computer screen. This task was completed under two conditions: sleep-saturated (nine in bed hours/night for two nights) and sleep-restricted (four hours in bed/night for two nights). The descriptions were analysed for number of words spoken per minute, average number of words spoken per figure described, and average word length.

Results: Using mixed linear models, we found that sleep restriction significantly reduced speech speed by about 3.52 words per minute (p = 0.003) and decreased the number of words used per figure by 2.20 (p < 0.001), with no difference in word length (p = 0.261). Participants adapted their speech depending on whether the listener was a child or an adult by using fewer words per figure (1.71 fewer words, p < 0.001) and shorter words (0.02 fewer letters, p = 0.011) when speaking to a child. There was no difference in how quickly they spoke to a child compared to an adult (p = 0.136). Additionally, no interaction effects were observed between sleep condition and whether the listener was an adult or a child (all p > 0.570).

Conclusion: Although sleep restriction led to slowed speech, it did not significantly impair the tendency to tailor one's speech to different listeners. These findings suggest a resilience in the social cognitive processes involved in tailoring speech to different audiences following insufficient sleep.