Noise
A noisy sleep environment can impair sleep but the threshold for arousal varies according to the age of the subject and the stage of sleep.

Arousal is also more likely if the noise is of significance to the sleeper, such as the crying of a child to the mother. Children sleep through more noise than adults, and older subjects are readily aroused from sleep by noise unless they are deaf. The threshold for arousal increases from stage 1 through to stage 4 NREM sleep.

Arousal is therefore more likely later in the night after the initial episodes of stages 3 and 4 NREM sleep have been completed.

International standards for nocturnal environmental noise have been drawn up. In general they recommend a mean nocturnal noise level within the sleeping area of less than 40dB and the avoidance of peak levels greater than 60dB. Noise greater than 40dB frequently causes difficulty in initiating sleep and greater than 50dB often causes awakenings during the night. These thresholds, however, vary considerably between individuals, between nights and within each night, and with other environmental factors, such as room temperature. In young adults, for example, the threshold for arousal from stage 4 NREM sleep is around 100dB, but this falls to 70dB at age 70 years.

Adaptation to environmental noise occurs rapidly, and people living, for instance, close to railways, motorways and even airports may sleep soundly.

Measures to isolate sleepers from noise can be successful and include earplugs and double glazing.