Pain and discomfort
Both internal and environmental stimuli can cause arousals from sleep despite the pain threshold being increased during sleep. Chronic disorders such as rheumatoid arthritis and multiple sclerosis commonly cause arousals of this type, and while a hard bed may help to reduce backache, it also leads to more frequent changes of position during sleep which are associated with brief awakenings.

Temperature
A constant environmental temperature of around 18°C (65°F) is ideal for inducing and maintaining sleep and for the optimal balance of NREM and REM sleep. Both high and low environmental temperatures lead to fragmentation of sleep. Rapid eye movement sleep is particularly sensitive to temperature changes and is significantly shortened in cold environments.

REM sleep is facilitated by a rise, and NREM sleep by a fall, in core body temperature.

Physical exercise
Exercise promotes wakefulness not only at the time of the activity, but for around 3h afterwards. This alerting effect is present both in normal subjects and also in, for instance, those with narcolepsy.

Exercise early in the day may cause a phase advance, reducing the sleep latency on the next night. It increases the total sleep time and the duration of stages 3 and 4 NREM sleep, and may reduce the duration of REM sleep. Exercise within around 3h of the desired time of sleep onset may delay this and reduce melatonin secretion.