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Diurnal preference to eveningness might predispose to depression. There is preliminary evidence of infrared-A (IR-A) induced whole-body hyperthermia (WBH) in the treatment of depression. In this exploratory study with 19 adults, we investigated the effects of a 20-min exposure of water-filtered IR-A (wIRA) to the skin of back and buttock area, without inducing WBH, on mood and assessed the outcome by diurnal preference (#R19047, approval on 7 May 2019). The skin received irradiation with an integrated power of 102.4 W in the wavelength region of 550-1350 nm and a total dose of 123 kJ over the total area of 0.0483 m 2 . The diurnal preference was assessed with a 6-item version of the Morningness-Eveningness Questionnaire (mMEQ). The 40-item Profile of Mood States (POMS) questionnaire was used to assess total mood disturbance (TMD). Core temperature was measured 30 min before, during and 30 min after the irradiation. Skin surface temperature was measured on baseline and every two minutes during the irradiation. The TMD improved immediately after the exposure, and this positive effect lasted for 24 h (p = 0.001) as well as for 2 weeks (p = 0.02). Concerning the diurnal preference, the positive effect on mood was immediate and lasted for 24 h in evening types (p = 0.02) and for 2 weeks in morning types (p = 0.04). During the exposure, core body temperature was constantly lower in morning types compared to evening types. This study gives us new information on the effects of near-infrared radiation, without inducing WBH, through the skin on mood., Competing Interests: Declarations Conflict of interest The authors declare that they have no competing interests., (© 2024. The Author(s).)