Light strongly influences the circadian timing system in humans via non-image-forming photoreceptors in the retinal ganglion cells. Their spectral sensitivity is highest in the short wavelength range of the visible light spectrum as demonstrated by melatonin suppression, circadian phase shifting, acute physiological responses and subjective alertness. Here we tested the impact of short wavelength light (460nm) on sleep EEG power spectra and sleep architecture. We hypothesized that its acute action on sleep is of similar magnitude as the reported effects for polychromatic light at higher intensities, and significantly stronger than longer wavelength light (550 nm). The sleep EEG of 8 young men was analyzed after a 2-h evening exposition to blue (460 nm) and green (550 nm) light of equal photon densities (2.8x¹³ photons/cm²/s) and to dark (0 lux) under constant posture conditions. The time course of slow-wave activity (SWA; 0.75-4.5 Hz) across sleep cycles after blue light at 460 nm was changed such that SWA was slightly reduced in the first and significantly increased during the third sleep cycle in parietal and occipital brain regions. Moreover, blue light significantly shortened REM sleep duration during these two sleep cycles. Thus, the alterations in the dynamics of SWA and REM sleep durations were blue shifted relative to the three-cone visual photopic system and differently mediated by the circadian, non-image-forming visual system. Our results can be interpreted in terms of an induction of a circadian phase delay and/or repercussions of a stronger alerting effect after blue light persisting into the sleep episode.