According to recent reports, the number of individuals aged 65 and older is increasing exceptionally worldwide. In 2008, about 506 million belonged to this age group; however, researchers estimate that this will increase to 1.4 billion by 2040. This projected increase in older individuals suggests an increase in age-related health problems, particularly dementia and cognitive decline.
Studies: Sex hormones, sleep, and memory: Interrelationships across the adult female lifespan. Image credit: Photographee.eu / Shutterstock.com
The prevalence of dementia in people over the age of 60 appears to be doubling every 20 years, with the number of people with dementia estimated to reach 115.4 million by 2050.
According to the World Health Organization (WHO), Alzheimer’s disease (AD) is the most common cause of dementia. Aging is a natural factor that contributes to the occurrence of sleep disorders, memory disorders, and the risk of AD.
A recent study found that there is a link between sleep and cognitive impairment, as demonstrated by the incidence of AD and related dementias (ADRD). This study also reported that more fragmented sleep with too short or too long sleep duration increases the likelihood of ADRD.
Several pathological findings of AD have been observed in sleep-deprived individuals. For example, sleep deprivation is associated with increased interstitial fluid levels, CSF amyloid beta (Aβ) and tau concentrations, and Aβ deposition in the brain, with Aβ accumulation likely having a negative impact on sleep.
Biological gender also appears to play an important role in the incidence of AD, with females often at greater risk of ADRD compared to age-matched males. In addition, women experience a faster decline in cognitive function than men after a diagnosis of AD.
One recent meta-analysis revealed that women are at higher risk of sleep disorders and insufficient sleep throughout their lifetime compared to men.
In recent Frontiers in Aging Neuroscience researchers describe the relationship between biological sex, memory, sleep and hormones in relation to the risk of ADRD.
In the current study, the authors reviewed the extensive literature to identify any evidence linking female sex hormones to sleep and memory. Limited data were available on the effect of sex hormones on sleep and memory with respect to the menstrual cycle, menopause and pregnancy.
Some controversial data have been reported regarding the potential influence of fluctuating sex hormones on changes in memory and sleep. For example, some studies report that slightly increased levels of estrogen and progesterone are beneficial for memory and sleep. Other studies, on the other hand, denied these findings and stated that sex hormones have no significant effect on memory and sleep.
Estrogen and progesterone levels have been found to strongly influence sleep patterns, with a strong correlation between sleep and memory. Changes in sleep induced by sex hormones also influence the relationship between sex hormones and memory. In addition, fluctuations in estradiol and progesterone levels during different phases of the menstrual cycle affect sleep and memory.
Sleep spindles, which are defined as bursts of neural oscillations throughout the thalamus, affect long-term memory outcomes. This is because sleep spindles can increase synaptic plasticity and neurogenesis of the prefrontal and hippocampal cortices, which are key factors associated with long-term maintenance of cognitive health and next-day memory performance. These factors are also affected by fluctuations in estrogen and progesterone levels.
Another study found that a night’s sleep deprivation reduced progesterone levels but not estradiol levels in women. However, this finding contradicted another study that reported that female sex hormones did not change significantly after sleep deprivation. A possible interaction between sex hormones and sleep that affects memory is also hypothesized.
Several studies have linked sleep disturbances to clinical biomarkers of AD and ADRD. These studies further indicate that the female gender is most susceptible to AD, with fluctuations in estradiol and progesterone being associated with the occurrence of ADRD.
An association between several neuroprotective mechanisms, such as Aβ levels, glial cell function, and regulation of neuronal mitochondrial function, and a reduced risk of AD has been observed. For example, individuals belonging to older age groups show lower levels of sex hormones, which cause Aβ accumulation and neuroinflammation, both of which negatively affect sleep.
During menopause, hormonal changes affect sleep due to excessive neuronal activity. The difference in norepinephrine production between men and women may contribute to the increased risk of sleep disorders in women and subsequently AD.
The exact underlying mechanism behind sex differences in sleep and memory, as well as their association with ADRD, are not well understood. Longitudinal observational studies as well as experimental studies in animals and humans are therefore needed to better understand the interrelationship between sleep, sex hormones, memory and their implications for ADRD incidence.
Link to journal:
- Harrington, AY, Parisi, JM, Duan, D., et al. (2022) Sex hormones, sleep and memory: Interrelationships across the adult female life course. Frontiers in Aging Neuroscience 14. doi:10.3389/fnagi.2022.800278.
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