Examine bidirectional associations between daytime napping and nighttime sleep among pregnant individuals with insomnia disorder. We used baseline data from a randomized controlled trial of insomnia treatment during pregnancy (n = 116). Participants in their second or third trimester of pregnancy self-reported daytime napping and nighttime sleep parameters using a sleep diary and wore an Actiwatch-2 during the same 7-day period. Linear regression models, accounting for intraindividual correlation, were used to estimate associations between daytime napping and nighttime sleep parameters (duration, efficiency, quality, awakenings). Models were also stratified by trimester of pregnancy. Sixty-three percent of participants reported napping on at least 1 day. Among participants in the second trimester (65%), napping 15-59 minutes was associated with 6.3% greater self-reported sleep efficiency (95% confidence interval: 2.3, 10.2) and 0.5 units greater self-reported sleep quality (95% confidence interval: 0.0, 0.9) that night; napping 60+ minutes was associated with 0.6 hours shorter actigraphy-measured sleep duration (95% confidence interval: -1.0, -0.2). Napping was not associated with nighttime sleep overall or during the third trimester. Nighttime sleep parameters were not associated with napping duration the following day. Among pregnant individuals with insomnia in the second trimester, short napping duration was associated with higher self-reported sleep efficiency and quality; long napping duration was associated with shorter actigraphy-measured sleep duration. Additional research is needed to examine the interaction between nap duration and nap timing. In the future, these results may lead to more nuanced recommendations for daytime napping among pregnant individuals with insomnia disorder. Registry: ClinicalTrials.gov; Name: Treatment for Insomnia during Pregnancy; URL: https://clinicaltrials.gov/ct2/show/NCT01846585; Identifier: NCT01846585. Badon SE, Dietch R, Simpson N, Lyell DJ, Manber R. Daytime napping and nighttime sleep in pregnant individuals with insomnia disorder. J Clin Sleep Med. 2023;19(2):371-377.