To perform a within-subject analysis of the changes in
absolute and normalized rates of non-rapid eye movement (NREM) delta
power production across daytime naps and baseline sleep, specifically to
determine whether these rates change linearly or exponentially.
Subjects were studied in a 2-day protocol after screening for
sleep disorders. The first night was a baseline sleep recording that was
followed by a nap the next day at one of the following times: 900, 1200,
1500, 1800 with the nap order varied between subjects. Sleep was then
recorded on the postnap night with the baseline bedtime schedule. Subjects
completed all 4 nap conditions.
A 4-bed sleep research laboratory. Each subject slept in a separate
Subjects were young normal (YNs, mean age 22.4 yrs) and
elderly normal (ENs, mean age 71.4 yrs) adults.
Measurements and Results:
Electroencephalography was recorded
continuously during baseline, nap, and postnap sleep. Fast Fourier transform
and period-amplitude analysis and visual scoring were performed.
Delta power/min and period-amplitude integrated amplitude/min were significantly
lower in EN than YN subjects. Both measures declined linearly
across NREM periods and increased linearly across naps in the two age
groups. The linear slopes for absolute power and integrated amplitude
were significantly flatter in the elderly. However, when normalized to each
individual subject’s baseline rate of delta production, the slopes of the ENs
closely approximated those of the YNs.
The observation of linear changes in delta across NREM
periods and daytime naps contradicts the exponential assumptions of the
two-process model. The similarity of the normalized slopes shows that
waking and sleep exert the same proportionate change in NREM delta in
YNs and ENs. This raises the possibility that homeostatic drive is similar
in the two groups despite different rates of absolute delta production. The
data here, taken in association with other findings, lead us to hypothesize
that NREM delta is specifically homeostatic for the neuronal systems that