Effect of intermittent truck noise on percentage of deep sleep.

Sleep was divided into only two categories, "deep" and "shallow," the latter including waking, stage 1, and dreaming. For the sake of objectivity and precision the criterion was the level of alpha frequency and/or the audibility of spindles (from a speeded up tape recording of the EEG signals from frontal electrodes), which were timed by stopwatch. Twenty truck noises, presented every other night at peak levels of 65 dB (A-weighted), reduced deep sleep of 12, mainly young subjects by an average of 3%. Adaptation of the waking response occurs despite the separation of the noise nights by at least one quiet night. Adaptation to sleeping in the laboratory appears to continue for more than the one or two nights usually assumed.