I'm sleepy! As people in the modern era try to fit more activities and increasing responsibilities into their lives, how often do we hear this complaint, or even make the complaint ourselves? There just aren't enough hours in the day, it seems, and we compensate by cutting back on sleep. As the population ages, with people become more overweight and more subject to obstructive sleep apnea, the problem of getting enough rest is compounded.
We expect sleep deprivation to make us less alert. But one of the side effects of sleep loss is quite unexpected--both voluntary sleep restriction and disordered breathing during sleep result in insulin resistance. This is surprising on an intuitive level. Logically, we would expect that the less time we spend sleeping, the more time we would spend in being active and burning up extra calories. Many studies indicate that this is not the case.
Although most mammals sleep for a few hours at a time throughout the day, humans expect to get most of their sleep during a single seven to nine hour period. This entails a prolonged fast, and several mechanisms are present in human beings to enable this to occur. Cortisol is at a low level as sleep begins. Growth hormone is secreted to allow fatty acids that were stored during waking hours to be mobilized and used as fuel. During the first part of sleep, glucose levels increase because there is a decrease in the utilization of glucose in the brain and in the peripheral tissues. The increase in blood glucose is followed by an increase in insulin secretion. As sleep progresses, REM sleep causes the brain to use up some of the glucose, and the secreted insulin lowers the glucose levels further. The sleep cycle nears its end with cortisol levels starting to rise and continuing to do so until about 30 minutes after awakening, preparing the sleeper to face the challenges of the upcoming day.
Insufficient sleep or disrupted sleep interferes with this ordered hormonal cycle. In a review published in 2005, Spiegel et al. described the effects of sleep disruption on healthy adults. In sleep-deprived subjects, there was an increase in evening cortisol levels and in nighttime growth hormone concentrations. In the early part of the day, their glucose levels were higher and their insulin levels were lower. They also showed an increased appetite for food with a high carbohydrate content. Insufficient sleep is also associated with long-term weight gain. In light of that, another interesting finding was that sleep-deprived subjects saw a decrease in the satiety hormone leptin, and an increase in the appetite-stimulating hormone ghrelin.
Voluntary curtailment of sleep is one thing. Sleep disturbance can also occur as a result of obstructive sleep apnea (OSA). Obstructive sleep apnea is caused by the temporary collapse of soft tissues in the throat, resulting in the cessation of breathing many times during the night. The affected person may awaken with the sensation of not having rested properly, but be completely unaware that his breathing has been interrupted. If obstructive sleep apnea is suspected, the diagnosis can be made by polysomnography in a sleep lab.
As one might expect, obstructive sleep apnea also interferes with the sleep cycle. In 2002, Ip et al. showed that obstructive sleep apnea is also associated with insulin resistance, and that the fasting insulin level and insulin resistance both increased as the hourly number of apnea (no breathing) or hypopnea (very shallow breathing) episodes increased. Patients with obstructive sleep apnea have increased sympathetic (fight or flight) activity when they are awake as well as when they are asleep. The sympathetic hormone epinephrine causes glucose release and glucose synthesis, and its ongoing presence in people with obstructive sleep apnea could account for at least part of their observed increased in insulin resistance.
With all of that in mind, here are some suggestions for those who would like to do something about chronic sleep problems:
- If you aren't reserving enough time for sleep, remember Benjamin Franklin. "Early to bed and early to rise makes a man healthy, wealthy, and insulin-sensitive." (I might have made up that last part.)
- If you are having trouble sleeping, you may wish to consult this list of suggestions from the University of Maryland: Sleep Hygiene: Helpful Hints to Help You Sleep.
- If you have obstructive sleep apnea, there are several possible approaches including weight loss, oral appliances, continuous positive airway pressure (CPAP), and even surgery. Here is a discussion of some of the options from the Mayo Clinic.