Humans exhibit distinct risk preferences when facing choices involving potential gains and losses. These preferences are subject to neuromodulatory influence, particularly from dopamine and serotonin. As neuromodulators manifest circadian rhythms, this suggests decision making under risk might be affected by time of day. Here, in a large subject sample (N = 26,720), we found that risky options with potential losses were increasingly chosen over the course of the day. Using a computational modelling approach, we show this diurnal change in risk preference reflects a decrease in sensitivity to increasing losses, but no change was observed in the relative impacts of gains and losses on choice. This diurnal sensitivity, present across two different task designs, was robust to age and gender and present in both between- and within-subject analyses. Thus, our findings reveal a striking diurnal modulation in human decision making, a pattern with potential importance for real-life decisions that include voting, medical decisions, and financial investments.