Purpose: Critically ill patients are at risk of sleep deprivation. Sleep promotion should be routinely implemented. Passive body heating has the potential to promote sleep. It is not known whether it can be obtained with a bed bath in the intensive care unit. Methods: In this cross-sectional observational study, we compared the sleep of 30 adult critically ill patients in two sequential nights – without and with bed bath, recording potential intervening environmental and clinical factors. Additionally, we monitored the impact of baths on participants' body temperature, checking for correlations between changes triggered by bath and sleep parameters. Sleep was assessed with bispectral index and with Richards-Campbell Sleep Questionnaire. Environmental noise and luminance were measured with decibel and lux meters. Temperature was measured with iButtons sensors. Results: Patients had reduced total sleep time in both nights of observation (medians of 170 and 177 minutes), without reaching deep sleep. There was no difference in sleep between both nights of observation. We recorded high noise levels in both nights of observation (57dB) and bathing nights were associated with greater exposure to light. There was moderate correlation between epigastric temperature oscillation with bathing and sleep volume, but epigastric temperature predicts neither total sleep volume nor continuous sleep volume. Conclusion: In critically ill adult patients, nighttime bed baths do not correlate with sleep, but are associated with increased exposure to light. Post-bath epigastric skin temperature correlates with sleep volume but does not predict it.