Had a few queries after an interview with news.com.au that was understandably brief. They were focused on the current temperature and sleep problems. Here is a more detailed background.
Human skin is under the influence of environmental temperature but also is regulated by internal processes like the autonomic nervous system and blood flow. Information on skin temperature is conveyed to the brain temperature centres but also to areas that relate to regulation of sleep. “Warm blooded animals” maintain their core body temperature within a small range unlike “cold-blooded” who need the sun’s warm rays to keep active (no snakes in Ireland !!). So us furry creatures elevate our skin temperature during the sleep period through an increase in skin blood flow in combination with behavior that limits heat loss through insulation by creating a warm microclimate, like covering and curling up.
Experiments show that by warming certain parts of an animal’s brain (preoptic area of the anterior hypothalamus), sleep can be induced*. Conversely, mild skin warming has also been associated with sleep-like activity in various other parts of the brain. Therefore the effect of a mild increase in brain temperature may differentially drive different brain areas towards either a more sleep-like or a more wake-like firing pattern. It’s a balance that depends on the temperature in the environment, genetics, circulating hormones and a range of other factors such as aging – however in some cases mild increases in temperature will assist sleep while in others it may impair the ability to sleep. Ask anyone with hot flushes (called hot flashes in North America for some obscure reason). Hot flushes are a rapid and exaggerated heat dissipation response (oestrogen-withdrawal related), consisting of profuse sweating, peripheral expansion of blood vessels, and feelings of intense, internal heat…..and very disturbing to sleep.
In humans, people sleep best while they head towards the low point of their 24-h core (internal) body temperature rhythm (late evening, night) and perform best around its peak point (early evening). As a side point, that is why when you have a fever it often peaks in the early evening. Importantly, healthy people fall asleep more easily if their skin temperature or bed temperature is higher and perform better during the dips of their normal daytime skin temperature fluctuations. Healthy elderly people and demented elderly people complain more about daytime sleepiness when they have elevated daytime skin temperature.
Chris Gordon from the University of Sydney Faculty of Nursing and a group from our centre are currently exploring skin temperature and vigilance in healthy controls and patients with sleep disorders. They are researching the gradient between the temperatures measured at a distal (eg hand, lower arm) and a proximal site (upper chest) that correlates most strongly with measures of vigilance/alertness. In 1999, Anna Wirz-Justice, a Basel based researcher who claims New Zealand citizenship around rugby season, reported with her co-workers in the journal, Nature, that reducing this gradient by warming the feet induced sleep. Heating your feet and hands increases the degree of dilation of blood vessels in the skin of the hands and feet, which increases heat loss at these extremities, and is the best physiological predictor for the rapid onset of sleep. In contrast, very mild skin cooling enhances sustained task performance and the ability to maintain wakefulness
However before rushing for blankets, mittens and socks, it may be important to initially warm to neutralise the gradient but then you need to avoid increasing core temperature which may impair sleep continuity. The future lies in clever ways of doing this and using feedback loops involving skin temperature that may improve sleep in aging and sleep disorders such as sleep apnea or insomnia or even assist with keeping operationally-critical people like nuclear power plant operators awake.
In our advice to insomnia patients, we tell them to heat the body prior to sleep, for example, by exercising or taking a hot bath or sauna (if you have one !). Body heating activates heat loss mechanisms for a duration that outlasts the actual heat stress, and consequently may keep skin blood flowing higher for a few hours. The leading Dutch sleep researcher, Eus Van Someren, examines time lapse recording of humans sleeping and wonders whether people, during brief arousals from sleep, apply “behavioral thermoregulation” by kicking away the blankets or duvet to expose limbs to the cooler environment alternating with covering themselves to create a warm microclimate. He asks whether this nocturnal thermoregulatory behaviour differs between insomniacs and people without sleep complaints. He thinks so and has also found evidence that certain types (phenotypes) of insomnia patients have structural abnormalities in the parts of the brain lining (orbitofrontal cortex) that is essential for the evaluation of comfort. We are also pursuing this line of work in a project headed by Chris Gordon and Chris Miller in part supported by the CRC for Alertness, Safety and Productivity.
*For those academic readers, Cliff Saper and colleagues have an excellent overview of the interconnections of hypothalamus including temperature and sleep in the Dec 14 issue of Current Biology
Professor Ron Grunstein is a Sleep Disorder Specialist based in Sydney, who consults at the Woolcock Clinic and Royal Prince Alfred Hospital and undertakes research at the Woolcock Institute, University of Sydney.