Insomnia

Insomnia is the perception or complaint of inadequate sleep due to difficulty falling asleep, waking up frequently during the night, waking up early, or experiencing sleep that is non-refreshing, combined with negative effects on subsequent daytime functioning.¹

Factors that may contribute to insomnia include:

environmental factors
(e.g., change in timing of the light–dark cycle; uncomfortable, noisy, or hot/cold surroundings; stressful life events)
a primary sleep disorder
(e.g., sleep apnoea, periodic limb movement syndrome, circadian rhythm disorder, restless legs syndrome)
another medical/neurological/psychiatric condition
(e.g., depression, anxiety, pain or movement disorders)
medical treatment, alcohol, or dietary stimulants such as caffeine
behavioural processes
(e.g., hyperarousal, which involves an increased metabolic rate; conditioning, where patients convince themselves that they will not be able to sleep)
increasing age.^2-4

Therefore, the underlying cause(s) of insomnia can be complex, as multiple contributing factors may act singly or in combination (Figure 1)

References:
1. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. Fourth edition text revision (DSM-IV-TR). Washington DC: American Psychiatric Association, 2000.
2. Vgontzas AN, Bixler EO, Lin HM, et al. Chronic insomnia is associated with nyctohemeral activation of the hypothalamic-pituitary adrenal axis: clinical implications. J Clin Endocrinol Metab 2001; 86 (8): 3787–3794.
3. Perlis ML, Giles DE, Mendelson WB, et al. Psychophysiological insomnia: the behavioural model and a neurocognitive perspective. J Sleep Res 1997; 6 (3): 179–188.
4. Kandel ER, Schwartz JH, Jessell TM. Principles of neural science. ^© The McGraw-Hill Companies Inc, 2000.
5. Szuba MP, Kloss JD, Dinges DF (Eds). Insomnia. Principles and management. © Cambridge University Press, 2003.