What’s more important sleep quantity or sleep quality?
When we speak about health benefits of sleep, we tend to concentrate on sleep duration more than the actual quality of sleep. But quality of sleep is as important as its quantity, and recent research has been digging into various aspect of sleep disturbances trying to uncover their implications on our longterm health.
New study published in the Journal Neurology last month found that people who have irregular sleep patterns may have a higher risk of dementia than those who have more regular sleep routines. This is a rather new angle of looking at sleep: where we are not focusing on the overall sleep quality (often subjective), and not insomnia or sleep wakefulness, but examining the impact of varied bedtime times and sleep wakefulness patterns over a length of time.
The study involved 88,094 people with an average age of 62 who were followed for 7 years. Participants’ sleep cycles were measured and analysed from the sleep tracking devices they wore at night. The researchers attributed a score of 100 to a person who sleeps and wakes at the exact same times each day. Meanwhile, a person who sleeps and wakes at different times every day would have a score of zero. After adjusting for age, sex and genetic risk of Alzheimer's disease, researchers found that those with the most irregular sleep patterns were 53% more likely to develop dementia than people in the middle group. Sleep regularity was estimated as the probability of a person being asleep or awake at any two time points 24-hours apart.
Over the past few years there’s been substantial evidence demonstrating that poor quality sleep in midlife or later is associated with cognitive decline and potentially causing neurodegenerative diseases, such as Alzheimer’s and dementia in old age. There are several factors at play here. Alzheimer’s and sleep seem to have a bidirectional relationship. In those patients diagnosed with Alzheimer’s disease over 60% have at least one diagnosed sleep disorder, either insomnia, or sleep apnea or other.
Sleep patterns in the elderly population demonstrate a reduction in deep sleep or slow-wave sleep. With increasing age it seems that our sleep architecture changes, and the proportions of each sleep stage change as well. There are problems with falling asleep and thus more people struggling with sleep latency; there are percentage changes in the sleep stages N1 and N2, these are designated for deep sleep, which the body priorities when sleep deprived.
It is believed that in the ageing population there’s a decreased sensitivity of the master clock - SNC to the environmental cues helping the body to adjust and regulate circadian rhythm to a natural 24-hour sleep-wake cycle. What’s more, melatonin levels decrease with ageing, and often display the levels similar to those of the daytime, thus affecting sleep onset dramatically. The ageing population have a higher number of different health conditions that may affect the quality of overall sleep and sleep patterns - gastrointestinal issues, such as heartburn or gastritis, high blood pressure, osteoarthritis, etc.
Deep sleep helps consolidate memory, so regular lack of deep sleep is linked to lower cognition and thinking.
One more factor of deep sleep acting as a guarding agent for dementia and Alzheimer’s prevention is that during slow-wave sleep, our body is activating a so-called cleansing system within the brain. It removes the buildup products and neurological toxins. The discovery of the glymphatic system by Maiken Nedergaard demonstrated that the brain similarly to the lymphatic system in the rest of the body has its own detoxing pathway - glial cells, which act as this intelligent waste clearance system. The glymphatic system connects with the lymphatic system of the rest of the body via the connective tissue that covers the CNS.
One of the harmful substances that the brain’s job is to clear during sleep is beta-amyloid. Beta amyloid are toxic proteins that build up inside the brain, lack of sleep is associated with increases in amyloid plaques. Researchers at the UC Berkeley’s Centre for Human Sleep Science found that even one night of sleep deprivation sees a significant increase in amyloid proteins.
Matthew Walker, PhD, the founder and director of the Center for Human Sleep Science, UC Berkeley, shows that our glymphatic system clears as much as 40 percent of the total amyloid-beta accumulation during deep sleep.
Good sleep quality is a well-recognised predictor of physical and mental health, overall vitality and longevity. While most of us can feel the results of one night’s poor sleep on the level of alertness, focus, impaired performance and reduced energy we have the following day, we ought to pay more attention and recognise sleep as the body’s essential protective mechanism against cognitive decline and age-related cognitive impairment. Improving sleep quality can protect against memory decline, boost cognitive performance, promote sharper thinking and reduce the risk of neurodegenerative diseases.