Numerous factors have been shown to increase sexual risk-taking-especially among gay and bisexual men (GBM), who remain disproportionately affected by HIV and STIs. We present three lines of evidence that highlight the need to consider a previously under-explored situational factor in sexual risk-taking: tiredness. While tiredness has been shown, in sleep science literature, to impair cognition, emotional functioning, and decision-making in a wide range of behaviors, it has yet to be considered in-depth as a risk factor in sexual behavior. Counter to the common-sense assumption that being tired should impede the performance of active, effortful behaviors such as sex, we propose that tiredness may actually increase sexual risk-taking. Analyzing data from an online survey of 1113 HIV-negative GBM, we found that sex with casual partners most commonly occurred at night, especially among younger GBM and those with an evening chronotype, and that sex without condoms more often occurred at or later than one's usual time of feeling tired (as was reported by 44.2% of men) than did sex with condoms (reported by 36.6%). We also found that tiredness can increase sexual desire in many GBM (endorsed by 29.9%), and increases the likelihood of engaging in receptive positioning in anal sex (endorsed by almost 40% of men with a versatile sexual positioning identity). These findings highlight the importance of considering tiredness as a situational risk factor in sexual health-especially among younger GBM-and of counting sex among the behaviors that can be adversely impacted by poor or overdue sleep.