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Research shows that of all insomnia causes, stress is the most common by far.Often, folk wisdom understands what science later proves. In the case of insomnia, one folk remedy is to drink a warm glass of milk. This has a calming effect, helping us to let go of the days stress and nod off to sleep.Researchers continue to confirm that the most common form of insomnia, referred to as primary insomnia, is stress related. Let's look at some of the research that bears this out. Corticotropin-releasing hormone impairs sleep.A study conducted at the Max Planck Institute of Psychiatry in Munich, Germany showed a reciprocal relationship between "growth hormone-releasing hormone (GHRH) and sleep-impairing corticotropin-releasing hormone (CRH)".Growth hormone-releasing hormone is a hormone that is secreted in the brain to signal the pituitary gland to release growth hormone. Growth hormone is normally secreted during Stage 4 deep sleep. 
Corticotropin-releasing hormone is another hormone that is released in the brain in response to stress. It signals the pituitary gland to release ACTH (adreno-cortical stimulating hormone) which in turn stimulates the adrenal cortex to release its stress hormones. Predominant among the stress hormones secreted is cortisol. Cortisol is an arousal hormone, increasing our state of arousal. The electroencephalogram was used to measure the brain waves of animals and humans in sleep. At the same time the levels of the above mentioned hormones and other brain chemicals were measured. The ratio between stress promoting CRH and growth hormone stimulating GHRH was shown to be important with regard to sleep patterns. A higher proportion of CRH is associated with insomnia or poor sleep.(1) Corticotropin-releasing hormone inhibits deep sleep."Growth hormone (GH)-releasing hormone (GHRH) stimulates GH and slow wave sleep (SWS) and inhibits cortisol, whereas corticotropin-releasing hormone (CRH) exerts opposite effects."(2)This study was also done at the Max Planck Institute of Psychiatry and revealed similar findings to the one mentioned above. Subjects were injected with GHRH or CRH, changing the ratio of these hormones in the brain. Injecting GHRH (growth hormone-releasing hormone) promoted deep sleep (slow wave sleep). Raising GHRH in proportion to CRH also inhibited cortisol secretion. Lower than normal morning cortisol is associated with primary insomina."Cortisol after awakening was significantly decreased in primary insomnia. Salivary cortisol at the time of awakening correlated negatively with the subjective estimation of sleep quality, i.e. a low salivary cortisol level directly after awakening correlated with a higher frequency of nightly awakenings."(3)
This study measured the level of cortisol in subjects upon waking in the morning. 14 of the subjects had a history of primary insomnia whereas another 15 subjects had healthy sleep. The subjects with a history of insomnia had significantly lower cortisol upon rising in the morning than those with normal sleep. This suggests that disruption of the diurnal cortisol rhythm (regulated by the hypothalamic-pituitary-adrenal axis) is causal to primary insomnia. This study measured the level of cortisol in subjects upon waking in the morning. 14 of the subjects had a history of primary insomnia whereas another 15 subjects had healthy sleep. The subjects with a history of insomnia had significantly lower cortisol upon rising in the morning than those with normal sleep. This suggests that disruption of the diurnal cortisol rhythm (regulated by the hypothalamic-pituitary-adrenal axis) is causal to primary insomnia. The difference between night owls and early birds found in the levels of cortisol."The present data support the idea that morning relative to evening chronotypes might show higher cortisol levels in the first hour after awakening. In sum, individual chronotype should be acknowledged as one further possible source of interindividual variability in the cortisol rise after awakening."(4)In this study subjects natural sleep patterns were assessed by Horne and Ostberg's Owl-and-Lark-Questionnaire. Owl's like to stay up late whereas larks go to bed early and wake up early. A "chronotype" is someone who by this questionnaire is distinctly either a morning or late night person. 112 healthy, active young men were tested and of these 9 were clearly identified as morning chronotypes and 29 as evening chronotypes. Cortisol levels were measured in the morning upon awaking. The morning chronotypes had significantly higher levels of cortisol upon waking. This supports the contention that the normal spike of cortisol in the morning serves as an arousal mechanism. It concurs with other research which shows that disruption of the normal cortisol rhythm is associated with disruptions of the normal sleep pattern. "We conclude that insomnia is associated with an overall increase of ACTH and cortisol secretion."(5)“We conclude that insomnia is associated with an overall increase of ACTH and cortisol secretion, which, however, retains a normal circadian pattern. These findings are consistent with a disorder of central nervous system hyperarousal rather than one of sleep loss, which is usually associated with no change or decrease in cortisol secretion or a circadian disturbance. Chronic activation of the hypothalamic-pituitary-adrenal axis in insomnia suggests that insomniacs are at risk not only for mental disorders, i.e. chronic anxiety and depression, but also for significant medical morbidity associated with such activation.”(5)This study compared the levels of two stress hormones in insomniacs verus persons with healthy sleep. The two hormones, ACTH and cortisol are secreted in response to stress and are a part of the body’s overall stress response. They cause hyperarousal of the nervous system. Levels of ACTH and cortisol were measured in eleven young insomniacs and thirteen healthy sleepers. Levels of ACTH and cortisol were significantly higher in all of the insomniacs. The authors of the study conclude “activation of the hypothalamic-pituitary-adrenal axis leads to arousal and sleeplessness in animals and humans”. Said simply, stress causes insomnia. The authors go on to say that insomniacs have increased risk not only for mental disorders (chronic anxiety and depression) but also other medical problems associated with chronic stress. This is why stress is called the silent killer. 
"We found increased evening and nocturnal plasma cortisol concentrations in patients with primary insomnia."(6)“We found increased evening and nocturnal plasma cortisol concentrations in patients with primary insomnia. Considering that both ageing and psychiatric disorders are commonly associated with sleep disturbances, our results implicate that elevated cortisol concentrations are a rather unspecific feature of disturbed sleep. Furthermore, our data revealed a strong positive correlation between evening cortisol secretion and the number of nocturnal awakenings in both insomniac patients and controls. Since nocturnal exposure to increased HPA activity promotes sleep fragmentation even in healthy controls, increased evening cortisol levels may be a crucial factor in inducing and maintaining sleep disturbances. We therefore propose a model of HPA dysregulation in insomnia. This model is based on the arousal theory of insomnia and the strong correlation between evening cortisol secretion and sleep fragmentation as a pathophysiological mechanism of a vicious cycle of insomnia.”(6)These researchers found that higher levels of cortisol in the evening and at night in patients with insomnia. They found that higher levels of cortisol (stress hormones) in the body were associated with waking up more frequently at night. They propose that insomnia is a result of nervous system arousal by the hypothalamic-pituitary-adrenal axis (stress response system of the body). “Recent research provides evidence for an interaction between sleep and the activation of the hypothalamic-pituitary-adrenal (HPA)-axis"(7)“Recent research provides evidence for an interaction between sleep and the activation of the hypothalamic-pituitary-adrenal (HPA)-axis, but detailed studies in patients are still missing. We investigated hourly evening and nocturnal plasma cortisol secretion and sleep in seven male patients with severe chronic primary insomnia and age- and gender-matched controls. Evening and nocturnal cortisol levels were significantly increased in patients. Evening cortisol correlated with the number of nocturnal awakenings in patients and controls.”(7)This study evaluated the level of the stress hormone cortisol in seven male insomniacs and healthy sleepers during the evening and at night. The study found that cortisol levels in the men with insomnia were significantly higher. The level of their cortisol during the evening hours was correlated to (predicted) the number of times they awakened at night. Night time noise disrupts cortisol and sleep.“This study thus showed that night time exposure to LFN may affect the cortisol response upon wake up and that lower cortisol levels after awakening were associated with subjective reports of lower sleep quality and mood.”(8)In this study subjects were subjected to low frequency noise (LFN) designed to reproduce the effect of being subjected to traffic noise while sleeping. Levels of cortisol (stress hormone) were measured each morning after either a quiet night or a night of traffic noise (LFN). It was found that subjects took longer to fall asleep when exposed to traffic noise. On the nights that subjects were subjected to traffic noise, in addition to having higher measured cortisol, they reported in the morning that they didn’t sleep as well and were in a poorer mood. This study underscores the benefit of good sleep hygiene which includes a quiet sleep environment. “The hypothalamic-pituitary-adrenal (HPA) axis plays important roles in maintaining alertness and modulating sleep."(9)“The hypothalamic-pituitary-adrenal (HPA) axis plays important roles in maintaining alertness and modulating sleep. Dysfunction of this axis at any level (CRH receptor, glucocorticoid receptor, or mineralocorticoid receptor) can disrupt sleep.”(9)These authors investigate the role of stress in two sleep disorders, primary insomnia and sleep apnea. They particularly investigate the role of CRH (corticotrophin-releasing hormone) in arousal (awakening) and insomnia. They state that disruption of the body’s stress system (hypothalamic-pituitary-adrenal or HPA axis) plays a role in primary insomnia and sleep apnea. They state that this disruption involves hyperactivation of the body’s stress system. References:(1) Steiger, A. Sleep and Endocrine Regulation. Front Biosci. 2003 May 1;8:s358-76(2) Steiger A, Antonijevic IA, Bohlhalter S, Frieboes RM, Friess E, Murck H. Effects of hormones on sleep. Horm Res. 1998;49(3-4):125-30 (3) Backhaus J, Junghanns K, Hohagen F. Sleep disturbances are correlated with decreased morning awakening salivary cortisol. Psychoneuroendocrinology. 2004 Oct;29(9):1184-91. (4) Kudielka BM, Federenko IS, Hellhammer DH, Wüst S. Morningness and eveningness: the free cortisol rise after awakening in "early birds" and "night owls". Biol Psychol. 2006 May;72(2):141-6. Epub 2005 Oct 19. (5) Vgontzas AN, Bixler EO, Lin HM, Prolo P, Mastorakos G, Vela-Bueno A, Kales A, Chrousos GP. Chronic insomnia is associated with nyctohemeral activation of the hypothalamic-pituitary-adrenal axis: clinical implications. J Clin Endocrinol Metab. 2001 Aug;86(8):3787-94. (6) Rodenbeck A, Hajak G. Neuroendocrine dysregulation in primary insomnia. Rev Neurol (Paris). 2001 Nov;157(11 Pt 2):S57-61. (7) Rodenbeck A, Huether G, Rüther E, Hajak G. Interactions between evening and nocturnal cortisol secretion and sleep parameters in patients with severe chronic primary insomnia. Neurosci Lett. 2002 May 17;324(2):159-63. (8) Waye KP, Clow A, Edwards S, Hucklebridge F, Rylander R. Effects of nighttime low frequency noise on the cortisol response to awakening and subjective sleep quality. Life Sci. 2003 Jan 10;72(8):863-75. (9) Buckley TM, Schatzberg AF. On the interactions of the hypothalamic-pituitary-adrenal (HPA) axis and sleep: normal HPA axis activity and circadian rhythm, exemplary sleep disorders. J Clin Endocrinol Metab. 2005 May;90(5):3106-14. Epub 2005 Feb 22. From Research on Insomnia Causes to Causes of Insomnia To Home Page.
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