Sleep problems are extremely common in industrialized countries and the possibility that diet might be used to improve sleep has been considered. The topic has been reviewed many times, resulting in the frequent suggestion that carbohydrate increases the uptake of tryptophan by the brain, where it is metabolized into serotonin and melatonin, with the suggestion that this improves sleep. An alternative mechanism was proposed based on animal literature that has been largely ignored by those considering diet and sleep. The hypothesis was that, as in the hypothalamus there are glucose-sensing neurons associated with the sleep-wake cycle, we should consider the impact of carbohydrate-induced changes in the level of blood glucose. A meta-analysis found that after consuming a lower amount of carbohydrate, more time was spent in slow-wave sleep (SWS) and less in rapid-eye-movement sleep. As the credibility of alternative mechanisms has tended not to have been critically evaluated, they were considered by examining their biochemical, nutritional, and pharmacological plausibility. Although high carbohydrate consumption can increase the uptake of tryptophan by the brain, it only occurs with such low levels of protein that the mechanism is not relevant to a normal diet. After entering the brain tryptophan is converted to serotonin, a neurotransmitter known to influence so many different aspects of sleep and wakefulness, that it is not reasonable to expect a uniform improvement in sleep. Some serotonin is converted to melatonin, although the exogenous dose of melatonin needed to influence sleep cannot be credibly provided by the diet. This review was registered in the International Prospective Register of Systematic Reviews (CRD42020223560).
Objectives Carbohydrate is the nutrient most commonly said to influence sleep: it is proposed that a high intake increases the uptake of tryptophan by the brain, where it is metabolized into serotonin and melatonin. As this mechanism depends on the relative amount of carbohydrate and protein, studies were reviewed where diets differed in these macro-nutrients. Methods The Web of Science and Medline were interrogated using terms related to carbohydrate and sleep. Papers were retained if two diets, differing in the percentage of calories coming from carbohydrate, had been contrasted using either polysomnography or actigraphy. Measures considered with polysomnography included sleep onset time; sleep efficiency; rapid eye-movement (REM) and slow wave sleep (SWS). Measures examined from actigraphy included: sleep efficiency; duration of sleep. Meta-analysis was conducted using Review Manager 5.2 (Cochrane) using a random-effects model. Results With the polysomnography measures six studies met the inclusion criteria. A lesser consumption of carbohydrate was associated with more SWS (SMD = 0.47; CI 0.06 – 0.88; P = 0.02; I2 = 0%) and less REM (SMD = – 0.47, CI –0.87– –0.07, P = 0.02, I2 = 0%). A lower intake of carbohydrate was also associated with a shorter time before falling asleep (P = 0.03; I2 0%). Sleep efficiency is the percentage of time spent asleep, relative to the total time in bed. Using polysomnography there was a trend for better sleep efficiency to be associated with with a lower intake of carbohydrate, although it just missed significance (P = 0.06). However, using actigraphy those eating less carbohydrate were more sleep efficient (SMD = 1.25; CI 0.35 – 2.15; P = 0.007; I2 = 0%). Conclusions No study had the high level of carbohydrate needed to raise tryptophan; rather studies had enough protein to reduce the uptake of tryptophan. A novel possibility is that blood glucose levels modulate sleep. Glucose metabolism varies; it is less during SWS and greater with REM. There are many reports associating the nature of sleep with glucose tolerance. As several hormones control glucose levels, some stimulated by the level of glucose, there is a need to consider diet hormonal interactions. As SWS is believed to be restorative and aid plasticity, increasing SWS with lower carbohydrate have may functional implications. Funding Sources No external funding.
During sleep, emotional memories are preferentially strengthened. However, most studies on sleep and emotional memory focus on comparing negative valence with neutral valence stimuli. This study compared the sleep-dependent memory effects for stories and images, each comprising negative, neutral, and positive stimuli. It was hypothesized that a sleep effect would be seen for negatively and positively valenced stimuli. A novel story memory task (comprising three stories), and photographs from the Nencki Affective
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