Sleep interference is a category of annoyance that has received much attention and study. Everyone has been wakened or kept from falling to sleep by loud, strange, frightening, or annoying sounds. Being wakened by an alarm clock or clock radio is common. However, one can get used to sounds and sleep through them. Possibly, environmental sounds only disturb sleep when they are unfamiliar. If so, sleep disturbance depends only on the frequency of unusual or novel sounds. Everyday experience also suggests that sound can induce sleep and, perhaps, maintain it. The soothing lullaby, the steady hum of a fan, or the rhythmic sound of the surf can induce relaxation. Certain steady sounds serve as an acoustical shade and mask disturbing transient sounds.
Common anecdotes about sleep disturbance suggest an even greater complexity. A rural person may have difficulty sleeping in a noisy urban area. An urban person may be disturbed by the quiet when sleeping in a rural area. And how is it that a parent wakes to a slight stirring of his or her child, yet sleeps through a thunderstorm? These observations all suggest that the relations between expo sure to sound and the quality of a night's sleep are complicated.
The effects of relatively brief noises (about three minutes or less) on a person sleeping in a quiet environment have been studied the most thoroughly. Typically, presentations of the sounds are widely spaced throughout a sleep period of five to seven hours. Figure 6.3.4 presents a summary of some of these observations. The dashed lines are hypothetical curves that represent the percent awakenings for a normally rested young adult male who adapted for several nights to the procedures of a quiet sleep laboratory. He has been instructed to press an easily reached button to indicate that he has awakened and has been moderately motivated to awake and respond to the noise.
While in light sleep, subjects can awake to sounds that are about 30-40 dBs above the level they can detect when conscious, alert, and attentive. While in deep sleep, subjects need the stimulus to be 50-80 dBs above the level they can detect when conscious, alert, and attentive to awaken them.
The solid lines in Figure 6.3.4 are data from questionnaire studies of persons who live near airports. The percentage of respondents who claim that flyovers wake them or keep them from falling asleep is plotted against the A-weighted sound level of a single flyover. These curves are for approximately thirty flyovers spaced over the normal sleep period of six to eight hours. The filled circles represent the percentage of sleepers that awake to a three-
Awakening from Light Sleep Single Noise
Overall Single Noise
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