This cross-sectional analysis of 2,909 older community-dwelling men suggests associations of sleep architecture with poorer cognition. As expected, severe nocturnal hypoxemia (any sleep time with SaO2<80%) was also associated with poorer cognitive function. There was, however, no associations between less severe hypoxemia (sleep time spent with SaO2<90%), AHI or arousal index and cognition.
Spending less percent of sleep time in REM and more in stage 1 was related to poorer cognitive function in this cohort of older men. Specifically, less percent of time spent in REM sleep was related to measures of executive function and attention, while more percent of time spent in stage 1 sleep was related to these two domains and to the global measure of cognition. REM sleep is thought to be important for procedural memory.10,11
While no studies on older adults have been preformed, small studies on young, healthy volunteers have found that subjects with higher amounts of REM sleep performed better on learning motor tasks and memory.4–6,8,9
It has been suggested that SWS is needed for consolidation of declarative memory, but our study found no association with SWS and cognition.10,11
One study examining SWS and cognitive performance among insomniacs found an association with lower levels of SWS and worse performance, but no such relationship among normal controls.32
Changes in both sleep staging and cognition may be partially attributed to changes in health status. While a number of health status confounders have been adjusted for here, there may be unmeasured confounders that have not been accounted for that affect the relationship of sleep staging and cognition. Studies examining the longitudinal relationship of change in sleep architecture with change in cognition are needed to further clarify the association. Future studies that quantify the delta power may have improved ability to detect potential associations between SWS and cognition.
In addition to the relationship of sleep stages and cognition, we also found an association between severe overnight hypoxemia (any sleep time with SaO2<80%) and our measure of attention, the DVT. Numerous studies focusing on those diagnosed with obstructive sleep apnea syndrome (OSAS) have found apneics had worse levels of cognitive function than those without OSAS.14,16,17,20
A review of past studies of the association of OSAS and cognition found the most consistent relationship to be in the domain of attention/vigilance, as we have seen with the association of the DVT and overnight hypoxemia.48
The mechanisms for reduced cognition have been speculated to include effects due to hypoxemia, sleep fragmentation with daytime sleepiness, and altered sleep architecture.14–16,18, 21,22,24,29
Animal studies have shown that experimental hypoxemia can cause a wide range of effects that impact brain function, including cholinergic nerve damage, oxidative stress and inflammation, which in turn, have been shown to be associated with spatial memory and attention deficits.49
An association between nocturnal hypoxemia and executive function tasks has been observed in other studies.15,16,24
It has been recently reported that intermittent hypoxemia is associated to altered cerebral blood flow and reduced verbal memory.50
Our findings support a role for overnight hypoxemia in the pathophysiology of cognitive deficits associated with SDB. Indeed, we see this in the extremes, with worse average completion scores on the DVT for those men with any sleep time spent with SaO2
<80%. However, we did not observe associations between AHI or arousal index and cognition, suggesting that in older adults cognitive impairment is more strongly associated with nocturnal hypoxemia than the frequency of respiratory disturbances or arousals. In contrast to these findings, an association between SDB and cognition has been described among elderly community-dwelling women in the Study of Osteoporotic Fractures (SOF).30
In both the SOF and MrOS studies there was no association seen between SDB and the Trails B test, but among the SOF women there was an association seen between SDB and a global measure of cognition, the 30-point Mini-Mental State Exam (MMSE). These differing results found may be related to gender, or to the advanced age of the women in the SOF study. We did stratify by age in the current study and found no association between SDB and cognition among men 80 years or older. The 3MS test which was used in the MrOS study was developed to improve upon the MMSE test by extending the ceiling and floor of the test and to sample a wider range of cognitive abilities, with the MMSE test being restricted in its range of difficulty.40
Perhaps the differing results are due in part to the contrast in instruments used to test global cognition.
This study has several strengths. The study had a large population of community-dwelling older men who were not selected for inclusion based on sleep problems or cognitive impairment. The predictors were gathered objectively with in-home PSG. There were 3 measures of cognition which tested different domains, including a global assessment, executive function and vigilance. The data collected allowed for adjustment for multiple potential confounding factors, with results suggesting these associations were not explained by other covariates including depression, comorbidities, medication use, education, or lifestyle.
This study also had several limitations. The findings may not be generalizable to populations other than community-dwelling older men. Causality cannot be established due to the cross-sectional study design. The association of cognition and sleep staging may be bi-directional, so further research investigating direction of association on incident cognitive decline is needed. Adjustment for numerous covariates was performed, but there may be unmeasured confounders that could affect the results. None of the cognitive tests used specifically measure declarative memory, which is thought to be related to sleep staging.
These cross-sectional findings suggest an association with percent of sleep time spent in both stage 1 and REM with cognition, with less time spent in REM sleep and more in stage 1 sleep being related to poorer cognitive function. Severe nocturnal hypoxemia was related to vigilance, but no association was observed with the apnea-hypopnea index or arousals and cognition in these older community-dwelling men. Due to the lack of studies examining the relationship of cognitive function and sleep staging, these findings need to be replicated in other cohorts. Further study needs to be done to examine if these measures predict longitudinal cognitive decline.