Sleep and learning

Although the exact details are unknown, researchers believe that sleep is helpful in integrating and strengthening new memories. When I was a fellow, it was thought that this occurred mainly in dream (REM) sleep. Here is an article that studied the effect of slow wave sleep (SWS), a deep non-dream state, on memories related to fear. Apparently, studies have shown that memories can be reactivated during SWS by a reminder cue like odors or sounds experienced during SWS.

In this study, researchers used a conditioning procedure in humans to elicit a fear response. Specifically, they repeatedly paired a neutral sound with a mild electrical shock to the wrist of the participants. After enough pairings, the neutral sound would cause a measurable fear response in the participant, even without the electrical shock. In other words, the participant learned that the neutral tone meant the electrical shock was coming. After this conditioned learning, the participants slept for 4 hours because the first 4 hours of sleep are dominated by SWS. The second half of sleep is dominated by REM sleep, and the researchers wanted to isolate the effects of SWS.

One way to eliminate the learned fear response that has become paired with a conditioned stimulus like a neutral tone is to repeatedly expose the subject to the neutral tone but without the electrical shock. Over time, the neutral tone will stop causing the fear response as the subject learns that the electrical shock no longer is associated with the neutral tone. So in the study, the researchers played the neutral tone without the electrical shock during SWS. Results showed that this was able to attenuate the fear response. And this was compared to other subjects that received a different tone than the neutral tone and in those that received no tones at all. Finally, exposure to the tones in SWS did not appear to negatively affect sleep architecture or sleep quality.

So what does all this mean? Well in psychology, when someone has pathological fear or post-traumatic stress disorder, one effective treatment is to repeatedly expose the patient to what they fear, so that their fear response gradually attenuates. It's very challenging therapy for patients, because they have to consciously subject themselves to their fears. But, as this study suggests, maybe patients with pathological fear can be exposed to the feared stimulus during SWS, and not have to experience it consciously in order to reduce the fear response. Here's an analogy...it would be like going from having your appendix taken out while awake to having it taken out under anesthesia.

Obstructive sleep apnea and growth hormone

Growth hormone (GH) is secreted at night during our deepest levels of sleep. As the name implies, GH is involved in growth of our bodies. Too much GH as a child can result in a condition called acromegaly. The actor Richard Kiel, who played the character Jaws in two of the James Bond movies, has this condition. Too little GH can also negatively impact cardiovascular health as well as effect carbohydrate, fat, and lipid metabolism. GH is part of a group of hormones that include Insulin-like growth factor-1 (IGF-1) and IGF-1 binding proteins.

Obstructive sleep apnea (OSA) could reduce GH secretion due to the brief arousals from sleep, especially deep sleep. OSA can also affect GH levels by the repetitive oxygen level drops. It's been theorized that regular CPAP use may reverse the effects of OSA on GH. Previous studies have been inadequate to answer this question definitively. This study sought to determine if long term CPAP affects GH levels.

Researchers performed a randomized, double-blind, sham-controlled parallel group study. The participants were all men with moderate to severe OSA. They were randomized to receive either 12 weeks of real or sham CPAP. At the end of the 12 week period, all participants were put on real CPAP for another 12 weeks. The results showed that GH secretion was higher in those treated with real CPAP for 12 weeks compared to sham CPAP. Interestingly, the increase in GH was not due to improvements in the amount of deep sleep. Instead, the improvements in GH were associated with reduction in the oxygen level drops. Another interesting result was that the participants used real CPAP only 3.6 +/- 1.9 hours per night and the sham CPAP only 2.8 +/- 2.1 hours per night. Since most people sleep 7-8 hours per night, that amount of CPAP usage is low, yet the therapy still seemed to have a positive effect on GH.

Slow frequency sounds heard during sleep may improve memory

Slow wave sleep is the deepest level of sleep, occurring mostly in the first half of the night. It's called slow wave sleep because the waveforms 1-3 times per second - contrast this with non-deep sleep, where the waveforms occur 4-7 times per second. This article discusses research showing that sounds produced at the same frequency as slow wave sleep, and listened to while asleep, can enhance word associations. Sounds produced at different frequencies did not improve word associations.

The researchers also think these sounds may improve the overall quality of sleep - more studies are needed though.

Older age people may have memory problems from lack of deep sleep

This article discusses research about our aging brain, sleep quality, and memory. As we age, there is a reduction in volume in a part of our brain behind the middle of our forehead - the medial pre-frontal cortex (PFC). Sleep researchers think this area helps generate the deepest levels of sleep. As we age, the amount of deep sleep is reduced gradually. Many older people have little deep sleep. In this study, the researchers think that the older people who had less deep sleep than younger people, had reduced ability to remember new things. They think this effect is not so much about the volume reduction in the PFC, and more about the loss of deep sleep. Unfortunately, not much can be done to increase the level of deep sleep, whether you are old or young. Some medications have been shown to mildly increase deep sleep levels. Some researchers are apparently working with electrical stimulation of the brain - sounds interesting but I don't know more about it.

Acupuncture may increase deep sleep

This article discusses research showing that acupuncture may increase deep sleep. I have heard from some patients and friends anecdotally, acupuncture has helped their insomnia.

Deep Sleep, High Blood Pressure, and Elderly Men

There is an abstract in the journal Hypertension from August 2011 about the relationship between sleep stages and development of high blood pressure (a.k.a. HTN). There are three stages of sleep - light non-REM sleep, deep sleep, and REM (a.k.a. dream) sleep. Researchers studied 784 men greater than 64 years old who did not have HTN with an in-home sleep study. After a mean follow up of 3.4 years, 243 developed HTN. Analyses of the results demonstrated that developing HTN was associated with low oxygen levels, increased light non-REM sleep, and decreased deep sleep. After adjusting for age, nonwhite race, study site, and body mass index, only deep sleep was associated with developing HTN. The association was not reduced even after accounting for sleep duration, sleep fragmentation, and severity of sleep-disordered breathing (e.g. obstructive sleep apnea).

The researchers concluded that reduced deep sleep may contribute to adverse blood pressure in men. Unfortunately, the percentage of sleep that is deep sleep normally reduces with age in men and women. At the current moment, there is not a reliable way to increase deep sleep percentage without sleep deprivation or using medications.