This includes a variety of mechanisms that influence sleep but which are independent of the time spent awake and of circadian and ultradian rhythms. They modify the sleep–wake cycle according to changes in the environment which are significant for the individual. They are important but complex and ill-understood components of the sleep–wake control system. They have three main elements.

Behavioural factors
These include motivation, attention and other psychological responses to the environment. The voluntary choice of, for instance, deciding whether to take exercise or to move to a more stimulating situation influences the probability of remaining awake or falling asleep. The degree of conscious or subconscious awareness of the significance of different sensory inputs influences whether arousal from sleep occurs. Once aroused, the conscious brain is able to recognize the source of the stimulus and to respond accordingly.

The conscious decision either to try to fall asleep or to stay awake and knowledge of the clock time then influence whether or not the individual stays awake or falls asleep.

Social activity usually induces alertness but, through its association with physical activity and exposure to light, may have complex effects on sleep. Sleep is more likely after social interactions, and the resulting synchronization of sleep and wake patterns within a group or community may have a biological survival advantage.

Psychological factors
Mental stimulation before falling asleep makes doing so difficult, particularly if there are worries or anxieties which cannot be resolved. The ability to relax both mentally and physically affects whether or not sleep can be entered. Watching a television or video can be mentally stimulating because of the changing pictures, sounds and often exciting action, but equally they can induce sleep through boredom, immobility and low ambient light levels.

The degree to which the bedroom is mentally associated with sleep and how much it is, for instance, used as an office or a kitchen, affects the mental associations between going to bed and initiating sleep. Most people also find it more difficult to sleep in an unfamiliar environment, such as a hotel or sleep laboratory.

Reflex factors
The level or lack of sensory stimulation influences the sleep–wake state, and can be broken down into the following elements.