PROJECT SUMMARY Problematic cannabis use rates have increased in ’emerging adulthood’ (age 18-25), an age also characterized by high rates of insomnia and the developmental peak in a tendency toward eveningness (preference for later sleep timing). Cannabis-sleep relationships are increasingly recognized as pertinent for understanding risk for cannabis misuse, particularly given the high prevalence of cannabis use as a sleep aid. Insomnia and an evening chronotype are risk factors for cannabis use, and cannabis use affects sleep, but the mechanisms underlying these cannabis-sleep relationships are poorly understood. Experimental rodent studies implicate cannabis effects on homeostatic sleep drive (increased) and circadian photosensitivity (reduced), although some effects are evident only in males. However, these mechanisms have not been sufficiently investigated in humans, with scant research using the sleep EEG measures necessary to assess homeostatic sleep drive in relation to cannabis use, and no published studies examining cannabis effects on circadian photosensitivity. In response to RFA-DA-24-021, we propose to study the effects of cannabis use and discontinuation on sleep and circadian characteristics, including sleep physiology and circadian photosensitivity, in 60 emerging adults (18-25 y/o; 50% female) reporting regular cannabis use. We will examine participants throughout a 1-week Baseline period of regular cannabis use followed by a 4-week Cannabis Discontinuation reinforced by a contingency management protocol. Homeostatic sleep drive will be assessed at home using sleep EEG headbands during Baseline (on both nights of cannabis use and non-use), as well as during early and extended abstinence during the Discontinuation. Circadian photosensitivity and circadian phase will be assessed in the laboratory at Baseline and after Discontinuation. A Control group of 30 age- and sex-matched healthy-sleeping emerging adults without current cannabis use will undergo all Baseline procedures. Aim 1 will examine the effects of cannabis use and discontinuation on the homeostatic sleep drive. Aim 2 will examine the effects of cannabis use and discontinuation on circadian photosensitivity and phase. An exploratory aim will examine sex differences in cannabis effects on sleep, circadian rhythms, and light responsivity.