STUDY OBJECTIVES: To investigate the amplitude and phase of the circadian rhythm of core body temperature (CBT) via the continuous measure of the gastrointestinal temperature in participants with idiopathic hypersomnia (IH), non-specified hypersomnia (NSH) compared to healthy controls (HC) in a constant routine standardized bedrest (BR) protocol. METHODS: Consecutive participants evaluated in a National Reference Center for Rare Hypersomnias benefited from an extensive evaluation with one-night polysomnography, followed by a modified Multiple Sleep Latency Test (mMSLT), and a continuous 32-hour BR recording in standardized conditions. CBT was recorded via a telemetry pill (e-Celsius) during the BR, modeled by a Cosinor, with extraction of MESOR, amplitude, and phase. Participants with IH, diagnosed according to the International Classification of Sleep Disorders-3, were compared with participants with NSH (complaint of hypersomnolence but normal mMSLT and BR), and HC. Participants were divided into four groups based on their mMSLT mean sleep latency (mMSLT+,≤8 minutes) and their BR total sleep time (BR+, ≥19 hours). RESULTS: A total of 108 participants (80% women, 28.3 ± 7.8 years old) were included in the analyses, 81 IH (83% women), 16 NSH (75% women), and 11 HC (64% women). Cosinor amplitude and phase of CBT did not differ between IH, NSH, and HC, nor in the subgroup analysis (37 BR+/mMSLT+, 35 BR+/mMSLT-, 9 BR-/mMSLT+, 27 BR-/mMSLT-). No difference in chronotypes was observed between groups. Women had a greater MESOR and reduced CBT amplitude compared to men. CONCLUSIONS: The circadian rhythm of CBT showed no difference in amplitude or phase between IH, NSH, and HC, and was not related to prolonged sleep time or objective daytime sleepiness.