Study objective: 

Slow wave sleep (SWS) is the most important part of sleep, yet it decreases with aging. Homeostatic processes couple changes in waking to changes in slow wave sleep. Sleep pressure accumulates in the course of the day, inducing a wide variety of molecular changes in the brain, which are reversed during SWS. The quality and duration of SWS are compromised with increasing age, but the reason for these changes is not known.

Methods: 

We used the aging rat as a model to study why SWS decreases with aging. Young (4 months), middle-aged (12 months) and old (24 months) animals were implanted with electroencephalography (EEG) electrodes to record their sleep and wake behavior. Samples were simultaneously collected from the brain, and the changes in the concentration of sleep-inducing molecules were correlated with the behavioral states.

Summary of results: 

We found that the age-related decrease in EEG SWS correlates with a less pronounced increase in sleep pressure, as measured by various markers such as the latency to sleep and the accumulation of the sleep-inducing molecules nitric oxide (NO), adenosine (ADE) and brain-derived neurotrophic factor (BDNF).

Conclusions: 

Our results suggest that the reduction in SWS in aging is largely due to reduced sleep need (sleep pressure), reflecting attenuated wake promotion and changes in the molecular pathways regulating sleep in the aged brain.