Sleep

Although sleep requirements may not differ between young and middle-aged adults, older adults tend to have a more difficulty getting their required amount of sleep.^[8] Some studies have suggested that, rather than being an intrinsic part of the aging process, sleep deficiencies may be secondary to other illnesses in older people.^[9] Regardless of the cause, both short and long sleep durations have been associated with increased risk of cardiovascular disease and cancer mortality.^[10] Some researchers have speculated that negative health-associations with long sleep duration may be due to decreased sleep quality due to sleep fragmentation.^[10]

A few nights of subpar sleep will leave anybody groggy, irritable, and a bit lacking in brain function. A major mechanism by which one becomes sleepy is the buildup of adenosine levels in the brain, which are lower after getting adequate sleep. Based on the adenosine hypothesis, sleep debt (caused by excessive adenosine levels in the brain) could be paid back at some time in the future by getting better sleep. However, recent animal studies suggest that this might not be the case. Instead, extended periods of sleep deprivation may lead to lasting brain injury through increased oxidative stress and inflammation that can worsen neurodegeneration.^[11] Although it remains to be seen whether similar mechanisms are at work in humans, 5–10 days of sleep deprivation impaired cognition in humans, which did not fully recover after 3 or more days of adequate sleep.^[12][13] This suggests that extended periods of sleep loss may cause lasting, negative effects on the brain that may not be “paid back” with a few days of good sleep.

Sleep plays an important role in overall health. Not getting enough sleep, or not getting enough good-quality sleep, is associated with a higher risk of many negative health outcomes. The following are just some of the outcomes linked to poor sleep:

• Coronary heart disease. Sleeping less than 7 hours per night is associated with a higher risk of coronary heart disease (CHD) and an elevated risk of cardiovascular disease (CVD) mortality.^[14][15] Additionally, a genetic study found that a short sleep duration (6 or fewer hours per night) appeared causally related to a higher risk of CHD, high blood pressure, and heart attack.^[16]

• Obesity: Getting less sleep is associated with a higher risk of obesity.^[17][18] Consistent with this association, restricting sleep to just 4 hours per night has been shown to increase hunger and calorie intake and lead to weight gain (sometimes in as little as 5 days).^[19][20] Conversely, increasing sleep time from 5.9 to 7.1 hours per night in one clinical trial led to a reduction in caloric intake and a loss of body fat after 2 weeks.^[21]

• Alzheimer’s disease: Poor quality sleep and sleep problems are associated with a higher risk of Alzheimer’s disease, and a short sleep duration is linked to a faster rate of cognitive decline.^[22][23] This could be related to the glymphatic system, a biological drainage system active during sleep that seems to clear the brain of beta amyloid, a protein linked to Alzheimer’s disease.^[24][25]

• Type 2 diabetes: A meta-analysis of roughly one million people found that a short sleep duration(6 or fewer hours per night) was associated with an increase in the risk of type 2 diabetes.^[26] Additionally, a genetic study found that insomnia increased the risk of type 2 diabetes, suggesting a causal association between sleep problems and diabetes.^[27]

• Bone fracture: Sleep disturbances, insomnia, and a short sleep duration (less than 7 hours per night) are associated with a higher risk of breaking a bone, possibly by increasing the tendency to fall.^[28][29][30] A relationship between poor sleep, bone fractures, and low bone mineral density has also been observed in genetic studies.^[29][31]

• Depression: A short sleep duration (7 or fewer hours per night) is associated with a greater risk of depression. Having insomnia is also associated with a higher risk of developing depression^[32] and treating insomnia has been shown to improve symptoms of depression.^[33][34][35]

Health outcomes linked to poor sleep

It is thought that a carbohydrate-rich or a high-glycemic-index meal can affect sleep through its effects on tryptophan (an essential amino acid) levels.^[5] More specifically, high-glycemic-index carbohydrates cause a rise in insulin levels, which drive circulating large neutral amino acids (LNAAs)^[6] into muscles. Because LNAAs compete with tryptophan for transport across the blood-brain barrier, lower levels of blood LNAAs result in a higher tryptophan-to-LNAA ratio, so more tryptophan can travel to the brain. More tryptophan entering the brain leads to greater production of serotonin,^[7] and, finally, to the secretion of melatonin (a sleep-inducing hormone), resulting in sleepiness.

While there is a general consensus that exercising in the morning or afternoon benefits sleep, it’s often recommended to avoid exercising, especially at a high intensity, in the evening in fear of it negatively affecting sleep.

However, according to a meta-analysis published in 2021, an acute bout of high-intensity exercise performed 2–4 hours before bedtime does not disrupt sleep.^[36] In fact, performing high-intensity exercise 2 hours before bedtime tends to increase total sleep time (+16 minutes) and decrease the time it takes to fall asleep (−5 minutes). However, longer duration high-intensity exercise (>30–60 minutes) may decrease rapid-eye movement sleep to a small extent (−3%).

In a 7-week study in elite youth soccer players, an evening high-intensity exercise session did not affect sleep quality and slightly increased sleepiness at bedtime, compared to nights where no exercise was performed.^[37]

In sum, high-intensity evening exercise does not appear to negatively affect sleep. Careful consideration should be given to pre-workout supplements when exercising in the evening, as products containing caffeine can negatively affect sleep.