(Neurology Thematic Programme of the Semmelweis University)
(the Institute of Behavioural Sciences, Semmelweis University)
Homeostatic and circadian processes play a pivotal role in determining sleep structure,
timing, and quality. In sharp contrast with the wide accessibility of the electroencephalogram
(EEG) index of sleep homeostasis, an electrophysiological measure of the circadian
modulation of sleep is still unavailable. Evidence suggests that sleep‐spindle frequencies
decelerate during biological night. In order to test the feasibility of measuring
this marker in common polysomnographic protocols, the Budapest‐Munich database of
sleep records (N = 251 healthy subjects, 122 females, age
range: 4–69 years), as well as an afternoon nap sleep record database (N
= 112 healthy subjects, 30 females, age range: 18–30 years) were analysed by the individual
adjustment method of sleep‐spindle analysis. Slow and fast sleep‐spindle frequencies
were characterised by U‐shaped overnight dynamics, with highest values in the first
and the fourth‐to‐fifth sleep cycle and the lowest values in the middle of the sleeping
period (cycles two to three). Age‐related attenuation of sleep‐spindle deceleration
was evident. Estimated phases of the nadirs in sleep‐spindle frequencies were advanced
in children as compared to other age groups. Additionally, nap sleep spindles were
faster than night sleep spindles (0.57 and 0.39 Hz difference for slow and fast types,
respectively). The fine frequency resolution analysis of sleep spindles is a feasible
method of measuring the assumed circadian modulation of sleep. Moreover, age‐related
attenuation of circadian sleep modulation might be measurable by assessing the overnight
dynamics in sleep‐spindle frequency. Phase of the minimal sleep‐spindle frequency
is a putative biomarker of chronotype.
