The defining characteristics of insomnia are widely recognized as difficulty falling asleep, difficulty staying asleep, and sleep that is non-restorative [1]. Insomnia is among the most common health complaints: about 10% of the adult population complains of a chronic insomnia problem [2]. With aging, increasingly disturbed sleep and less satisfaction with sleep quality are reported [3]. This common problem has wide ranging physiological, cognitive, and behavioral consequences including higher healthcare utilization [4–6]. Current major treatment options for insomnia (hypnotic medications and non-pharmacological behavioral interventions) suffer side effects and shortcomings.
Thermoregulation plays a key role in promoting and maintaining sleep. At night, core body temperature (CBT) drops while distal skin temperature (DST) increases. It was previously believed that the nighttime drop in CBT was the most important promoter of sleep. However, recent research has shown that it is in fact the increase in DST (with net body heat loss owing to the large distal skin surface area) which is associated with an increase in sleepiness, whereas a decrease in DST (with resulting net body heat retention) is associated with a decrease in sleepiness [7]. The amount of distal vasodilation, as measured by the distal-proximal skin temperature gradient (DPG), is more predictive of sleep onset than subjective sleepiness ratings, CBT, or dim light melatonin onset. In fact, “the degree of dilation of blood vessels in the skin of the hands and feet, which increases heat loss at these extremities, is the best physiological predictor for the rapid onset of sleep” [8].
The link between distal skin warming and sleep propensity is further strengthened by the fact that warm water immersion of hands and feet has been found to decrease sleep onset latency (SOL) and pre-sleep warm baths have long been prescribed as an insomnia treatment. In a recent study, we used a multiple sleep latency test (MSLT) to perform multiple nap trials throughout the day, with the participants’ hands and feet immersed in warm water prior to each nap. We found that both mild and moderate warming of the hands and feet prior to a nap significantly reduced SOL compared to a baseline MSLT without warming [9]. We also previously conducted a trial of temperature biofeedback for insomnia treatment in which we demonstrated SOL reduction using muscle relaxation techniques to induce distal vasodilation, increase blood flow to the extremities, and modulate temperature of hands and feet [10]. Additionally, it has been shown that regardless of circadian variation throughout the day, finger temperature shows a rapid increase immediately before sleep onset [11]. Lastly, people with primary vascular dysregulation (a condition caused by abnormal vasoconstriction that results in cold hands and feet) exhibit significantly increased SOL and greater difficulty falling asleep following nocturnal arousal [12]. Thus, some presentations of insomnia may be secondary to distal vasodilation failure.
The motivation for an active distal limb warming device as a treatment for insomnia is based on the established functional link between distal vasodilation and sleep induction [13]. Somewhat counterintuitively then, heating of hands and feet can induce distal vasodilation, promote net body heat loss, and facilitate sleep onset [14, 15].
The Effects of Environmental Thermal Condition on Transitional Skin Temperature of Peripheral Parts of Human Hands and Feet during Exercise.
