STUDY OBJECTIVES: Visual stimulation (VS) at 40 Hz is being tested as a non-invasive approach against dementias such as Alzheimer's disease. Applying it during sleep could increase the convenience, duration, and efficacy of stimulation. Here, we tested the feasibility of 40 Hz VS during sleep in a proof-of-concept study. METHODS: Thirty healthy participants underwent one control and one experimental night of polysomnography at the sleep laboratory. 40 Hz VS was delivered in wakefulness (W), NREM sleep stages 2 and 3, and REM sleep. RESULTS: As expected, 40 Hz EEG spectral power was increased in all four stages in the experimental condition, compared to control. It was highest in W and similar across NREM 2, NREM 3, and REM, with large and medium effect sizes, respectively. Steady-state visually evoked potential (SSVEP) analyses in the time domain confirmed the specificity of the effect. Secondary analyses revealed that the intervention did not impair objective and subjective sleep quality beyond the first-night effect. CONCLUSIONS: 40 Hz VS during sleep effectively evoked neuronal gamma activity at stimulation frequency without degrading sleep quality, supporting the feasibility of this approach. These findings lay the groundwork for optimizing gamma-band sensory stimulation as a tool to causally study cognitive functions and as a scalable, non-invasive intervention against dementias.