In order to sleep, we need to be physiologically sleepy and physically and mentally relaxed.
(1) Factors that undermine being sufficiently sleepy
An irregular sleep schedule.
A sleep window that is often too big (e.g., >9 hours) or often too small (e.g., <7 hours).
Too much exposure to blue light during the evening or at night.
A lack of physical activity during the day.
Napping during the day (>20 minutes) or dozing in the evening.
(2) Factors that undermine being sufficiently relaxed
Not having a wind-down period before getting into bed (>30 minutes).
Not becoming sufficiently relaxed mentally and physically prior to getting into bed.
Lying in bed for extended periods while frustrated and/or worried, particularly being worried about not getting enough sleep.
Inaccurate or unhelpful beliefs about sleep and insomnia.
Trying too hard to fall asleep.
Psychological dependency on sleep medication.
(3) Other factors
Substances that interfere with sleep (e.g., caffeine, alcohol, some medications)
A non-optimal sleep environment (e.g., not cool, dark, quiet, and comfortable).