When self-regulatory resources are depleted, people tend to act more on ‘autopilot’, with minimal forethought. It follows that when sleepy, people should be more likely to act habitually, based on learned cue-behaviour associations that trigger behaviour automatically when the cue is encountered. This ecological momentary assessment study investigated whether, over the course of a week, between-person differences and momentary within-person variation in daytime sleepiness were associated with the reported habit strength of behaviours. Participants (N = 105, 71% female, M age = 35) completed a baseline assessment of sleep quality and, six times daily over seven days, momentary assessments in which they reported the habit strength of the behaviour they were doing when prompted and their momentary sleepiness. Multilevel modelling revealed that people who were more sleepy than others were not more or less likely to act habitually, but on occasions when people were more sleepy than was typical for them, the behaviour they were engaging in tended to be more habitual (Pseudo-R² = 2%). Our results suggest that sleepiness causes people to rely on non-reflective processes such as habit to regulate their behaviour. Interventions should promote habit formation for desirable behaviours so that when people are sleepy, they can rely on the efficiency of habits to ensure they continue to enact wanted behaviours. Conversely, interventions promoting behaviours that require deliberative thought might encourage performance during times of day when people are more alert.