Clinical reviewThe relationship between insomnia and body temperatures
Section snippets
Insomnia
Insomnia is a very generic term with the criteria including difficulty initiating sleep, maintaining sleep, and early morning awakening, accompanied by some form of daytime impairment (e.g., fatigue).1 This review will focus on these three areas of sleep difficulties and the relationship with body temperatures. The International Classification of Sleep Disorders (ICSD-2)1 elaborates several categories of insomnia primarily on the basis of its etiology and contributing causes such as
Body temperature and circadian rhythm timing
Periodic circadian (24-h) variations or rhythms are ubiquitous in the human body and include physiological and behavioral rhythms of core body temperature, melatonin secretion, sleep propensity and subjective alertness. The sleep–wake cycle and body temperature rhythms have a stable internal phase relationship in normally entrained conditions with the timing of sleep highly correlated with the timing or phase of the circadian body temperature rhythm.2 The temperature and melatonin circadian
Sleep onset insomnia and delayed sleep phase disorder
Sleep onset difficulties may occur when an individual's temperature rhythm is timed relatively late with respect to their attempted sleep period. In these cases the delayed temperature rhythm may result in a delay of the evening wake maintenance zone such that it coincides with the attempt to sleep. The evening wake maintenance zones of normal and sleep onset insomniacs are illustrated in Figure 2. Some studies have found that individuals experiencing sleep onset insomnia have a relatively
Early morning awakening insomnia and advanced sleep phase disorder
The ICSD-2 nosology describes advanced sleep phase disorder (ASPD), or earlier terminology ‘syndrome’, as a disorder in which the major sleep period is early (advanced) with respect to the desired sleep/wake period.1 Individuals with ASPD have overwhelming early evening sleepiness, an early sleep onset and early morning awakenings. For example, bedtimes could be as early as 19:00 and wake times at 03:00. If the person tries to obtain more sleep in the morning by staying in bed, frequent
The treatment of insomnia by phase shifting (re-timing) circadian rhythms
If treatments that appropriately shift the rhythms of sleep onset and early morning awakening insomnia are efficacious, it would support the prediction that temperature rhythm de-synchrony contributes to the etiology of these two insomnias. Initial studies have demonstrated that bright light can phase shift circadian rhythms,30, 31, 32 with bright light presented before the CBTmin delaying the circadian rhythm and bright light administered after the CBTmin advancing the rhythm.33, 34, 35, 36, 37
Insomnia from hyper-arousal
Independent of circadian factors, it is clear that insomnia can arise from a wide range of stimuli that promote arousal. These include environmental (e.g., noise, elevated ambient temperature), social (e.g., poor sleep habits, shiftwork), psychological (e.g., conditioned responses, anxiety) and physiological (respiratory impairment, restless legs, pain) stimuli. At a broad conceptual level it is generally held that such stimuli lead to a ‘hyper-aroused state’ that is the immediate cause of poor
Skin temperature's role in thermoregulation
We have already described the relationships between core body temperature and poor sleep. However, a review of insomnia and temperature would be incomplete without reference to the role of skin temperature in thermoregulation. The regulation of core body temperature occurs as a combination of heat production and heat loss.3 When heat production is greater than heat loss, core body temperature increases and, conversely, when heat loss exceeds heat production, as in the late evening, it
Skin temperature and the sympathetic nervous system
Heat loss from distal skin areas occurs most rapidly when AVAs are maximally dilated.4, 77 Conversely, when vasoconstriction occurs, the amount of blood flow is restricted causing distal skin temperature values to decrease towards ambient air temperature (e.g., 20–22 °C in a laboratory). Vasomotor activity of AVAs is directed primarily from sympathetic nervous system constrictor neurons. When these neurons are activated, smooth muscles around AVAs constrict causing vasoconstriction, sometimes to
Does insomnia limit the down regulation of core temperature as a result of impaired distal skin vasodilation?
Good sleepers are typically known to be relaxed when attempting sleep, and manage to fall asleep within a reasonable amount of time (e.g., within 10–15 min). In contrast, insomniacs take longer to fall asleep initially and following awakenings from sleep.15, 50, 56, 58 Unlike good sleepers, insomniacs are not relaxed, even to the point of being anxious when attempting sleep.58, 94 This view has been supported very recently with the finding that the normal reduction of cognitive arousal and
Supporting research
The first study to investigate the skin temperatures of insomniacs attempting sleep was conducted in 1979 by Brown97 who found that insomniacs did show increases in toe skin temperature when attempting sleep, but that sometimes there was no observable change. Yet, when toe temperature increases were observed, they were more variable, and took twice as long to reach the same amount of temperature change compared to good sleepers. Freedman and Sattler56 found that compared to good sleepers
Conclusion
Insomnia subtypes may be differentiated in terms of the aspect of sleep that is the predominant chronic difficulty (difficulty falling asleep, waking frequently for long periods during the night, early final awakening before sufficient sleep is obtained). There appear to be specific abnormalities of body temperature associated with some of these subtypes. Difficulty initiating sleep at a normal time (sleep onset insomnia) is associated with a circadian core body temperature rhythm that is
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