Autonomic dysregulation and sleep homeostasis in insomnia.

STUDY OBJECTIVES: Insomnia is common in older adults, and is associated with poor health, including cognitive impairment and cardio-metabolic disease. Although the mechanisms linking insomnia with these comorbidities remain unclear, age-related changes in sleep and autonomic nervous system (ANS) regulation might represent a shared mechanistic pathway. In this study, we assessed the relationship between ANS activity with indices of objective and subjective sleep quality in older adults with insomnia. METHODS: Forty-three adults with chronic insomnia and 16 age-matched healthy sleeper controls were studied. Subjective sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), objective sleep quality by electroencephalogram spectral components derived from polysomnography, and ANS activity by measuring 24-h plasma cortisol and norepinephrine (NE). RESULTS: Sleep cycle analysis displayed lower slow oscillatory (SO: 0.5-1.25 Hz) activity in the first cycle in insomnia compared to controls. In insomnia, 24-h cortisol levels were higher and 24-h NE levels were lower than controls. In controls, but not in insomnia, there was a significant interaction between NE level during wake and SO activity levels across the sleep cycles, such that in controls but not in insomnia, NE level during wake was positively associated with the amount of SO activity in the first cycle. In insomnia, lower 24-h NE level and SO activity in the first sleep cycle were associated with poorer subjective sleep quality. CONCLUSION: Dysregulation of autonomic activity may be an underlying mechanism that links objective and subjective measures of sleep quality in older adults with insomnia, and potentially contribute to adverse health outcomes.

Neurostimulation techniques to enhance sleep and improve cognition in aging.

Sleep plays a critical role in the process of memory consolidation. In particular, during non-rapid eye movement (NREM) slow wave sleep, slow-oscillations, spindles, hippocampal sharp wave ripples, and their phase coupling are involved in the process of transferring and consolidating information recently encoded and temporarily stored in the hippocampus into long-term memory stored in the neocortex. There is evidence that aging and neurodegenerative conditions, in particular Alzheimer's disease, are associated with changes to this transient grouping of NREM oscillations. Therefore, methods to enhance sleep, particularly slow wave sleep, have the potential to improve cognitive performance. Transcranial electrical and magnetic stimulation have been shown useful to enhance sleep slow-waves and sleep-dependent memory consolidation, however there is need for more information regarding proper protocols of application and applicability and efficacy in patients with neurodegenerative conditions. Acoustic stimulation during sleep has been proven particularly effective in enhancing sleep slow-waves and spindles with associated improvement in overnight memory consolidation. More importantly, preliminary data indicate that similar results can be achieved in healthy older adults and those with amnestic mild cognitive impairment. Studies are needed to optimize the modalities of acoustic stimulation during sleep, which may vary based on age group or clinical disorder. Overall, non-invasive techniques of neurostimulation may represent a valid approach to mitigate cognitive decline associated with aging and neurodegeneration. Furthermore, they offer the unique opportunity to improve our understanding of the physiology behind sleep-dependent memory consolidation.

Acoustic enhancement of sleep slow oscillations in mild cognitive impairment.

OBJECTIVE: Slow-wave activity (SWA) during sleep is reduced in people with amnestic mild cognitive impairment (aMCI) and is related to sleep-dependent memory consolidation. Acoustic stimulation of slow oscillations has proven effective in enhancing SWA and memory in younger and older adults. In this study we aimed to determine whether acoustic stimulation during sleep boosts SWA and improves memory performance in people with aMCI. METHODS: Nine adults with aMCI (72 ± 8.7 years) completed one night of acoustic stimulation (stim) and one night of sham stimulation (sham) in a blinded, randomized crossover study. Acoustic stimuli were delivered phase-locked to the upstate of the endogenous sleep slow-waves. Participants completed a declarative recall task with 44 word-pairs before and after sleep. RESULTS: During intervals of acoustic stimulation, SWA increased by >10% over sham intervals (P < 0.01), but memory recall increased in only five of the nine patients. The increase in SWA with stimulation was associated with improved morning word recall (r = 0.78, P = 0.012). INTERPRETATION: Acoustic stimulation delivered during slow-wave sleep over one night was effective for enhancing SWA in individuals with aMCI. Given established relationships between SWA and memory, a larger or more prolonged enhancement may be needed to consistently improve memory in aMCI.

Strengthening sleep-autonomic interaction via acoustic enhancement of slow oscillations.

Slow-wave sleep (SWS) is important for overall health since it affects many physiological processes including cardio-metabolic function. Sleep and autonomic nervous system (ANS) activity are closely coupled at anatomical and physiological levels. Sleep-related changes in autonomic function are likely the main pathway through which SWS affects many systems within the body. There are characteristic changes in ANS activity across sleep stages. Notably, in non-rapid eye-movement sleep, the progression into SWS is characterized by increased parasympathetic activity, an important measure of cardiovascular health. Experimental manipulations that enhance slow-wave activity (SWA, 0.5-4 Hz) can improve sleep-mediated memory and immune function. However, effects of SWA enhancement on autonomic regulation have not been investigated. Here, we employed an adaptive algorithm to deliver 50 ms sounds phase-locked to slow-waves, with regular pauses in stimulation (~5 s ON/~5 s OFF), in healthy young adults. We sought to determine whether acoustic enhancement of SWA altered parasympathetic activity during SWS assessed with heart rate variability (HRV), and evening-to-morning changes in HRV, plasma cortisol, and blood pressure. Stimulation, compared with a sham condition, increased SWA during ON versus OFF intervals. This ON/OFF SWA enhancement was associated with a reduction in evening-to-morning change of cortisol levels and indices of sympathetic activity. Furthermore, the enhancement of SWA in ON intervals during sleep cycles 2-3 was accompanied by an increase in parasympathetic activity (high-frequency, HRV). Together these findings suggest that acoustic enhancement of SWA has a positive effect on autonomic function in sleep. Approaches to strengthen brain-heart interaction during sleep could have important implications for cardiovascular health.

Acoustic Enhancement of Sleep Slow Oscillations and Concomitant Memory Improvement in Older Adults.

Acoustic stimulation methods applied during sleep in young adults can increase slow wave activity (SWA) and improve sleep-dependent memory retention. It is unknown whether this approach enhances SWA and memory in older adults, who generally have reduced SWA compared to younger adults. Additionally, older adults are at risk for age-related cognitive impairment and therefore may benefit from non-invasive interventions. The aim of this study was to determine if acoustic stimulation can increase SWA and improve declarative memory in healthy older adults. Thirteen participants 60-84 years old completed one night of acoustic stimulation and one night of sham stimulation in random order. During sleep, a real-time algorithm using an adaptive phase-locked loop modeled the phase of endogenous slow waves in midline frontopolar electroencephalographic recordings. Pulses of pink noise were delivered when the upstate of the slow wave was predicted. Each interval of five pulses ("ON interval") was followed by a pause of approximately equal length ("OFF interval"). SWA during the entire sleep period was similar between stimulation and sham conditions, whereas SWA and spindle activity were increased during ON intervals compared to matched periods during the sham night. The increases in SWA and spindle activity were sustained across almost the entire five-pulse ON interval compared to matched sham periods. Verbal paired-associate memory was tested before and after sleep. Overnight improvement in word recall was significantly greater with acoustic stimulation compared to sham and was correlated with changes in SWA between ON and OFF intervals. Using the phase-locked-loop method to precisely target acoustic stimulation to the upstate of sleep slow oscillations, we were able to enhance SWA and improve sleep-dependent memory storage in older adults, which strengthens the theoretical link between sleep and age-related memory integrity.