Young, middle‐aged, and old subjects, 35 in all, were tested to determine the probability of disturbance of sleep, as judged by EEG records, by seven noises per night produced by a recording of a passing truck. There is roughly a linear relation with theA‐weighted noise level. Young and old people have nearly the same response while middle‐aged subjects are more sensitive by about 15 dB. This applies also to the probability of awaking. The probability of shifts in sleep to a shallower level does not appear to adapt in 24 successive nights with seven noises per night but the probability of waking drops to half value in about two weeks. Response increases with duration of the stimulus, at least over the limited range from fractions of a second to a minute.