Situational factors affect human behavior, among which dishonesty. Previous studies linked darkness to an increase in immoral behavior. Simultaneously, ‘dark mode’ is increasingly offered in software. Accordingly it is important to investigate the moral behavioral effects of dark mode. In a high-powered, pre-registered, and economically incentivized experiment, users of ‘dark’ and ‘light’ user interfaces were compared on honesty. Results showed no general effect of dark mode on honesty. However, dark mode promoted honesty in users that were awake for a longer period of time. As such, implementation of dark mode is safe and may even be encouraged.