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Background and Objective Human brain appears to be able to absorb, detect, and respond to low-level extremely low-frequency electromagnetic fields (ELF EMF). Controlled laboratory studies on human sleep under exposure to such fields are scarce. Only sleep-disturbing effects on nighttime sleep were reported for frequencies of 50/60 Hz, while lower frequencies (i.e., below 20 Hz) have not been tested. These frequencies overlap with the frequency range of the electroencephalographic (EEG) signal, and sleep researchers utilized the specific frequency patterns (1–15 Hz) for subdivision of the sleep-wake state continuum into wake and sleep stages. In particular, the deepest sleep stage (N3) is characterized by slow-wave EEG activity (1–4 Hz) and serves as an electrophysiological indicator of sleep restorative function. We examined the effects of exposure to a low-level ELF EMF on sleep architecture in afternoon naps. Methods Ten polysomnographic sleep characteristics obtained during two naps of 23 healthy volunteers, either with or without exposure to a 1 Hz/0.004 μT electromagnetic field, were compared. Results The effect of the 1 Hz/0.004 μT electromagnetic field exposure on amount of stage N3 was not significant despite the overlap of this intervention frequency with the frequency of slow waves. However, the total duration of sleep was significantly increased due to a significant increase of amount of stage N2. Thus, the exposure to an extremely slow (1 Hz) electromagnetic field did not reveal any sleep-disturbing effects. Instead, total duration of sleep increased due to increase of N2 amount. Conclusions A sleep-promoting action of exposure to the low-level 1 Hz electromagnetic field cannot be excluded.