How do preadolescents and older adults fall asleep? Spatiotemporal electrophysiological patterns of the sleep onset process during lifespan

Introduction/Objectives: Sleep and wakefulness do not reflectmutually-exclusive states, but instead represent local phenomena.Despite the massive age-related modifications occurring during lifespan,the electrophysiological (EEG) Sleep Onset (SO) features in preadolescenceand healthy aging have not been exhaustivelyinvestigated. Thus, we aimed to describe spatiotemporal EEG dynamicsof SO in preadolescents and older adults.Methods: The pre- vs- post-SO changes in the topography of EEGpower (1-Hz-frequency-resolution) and the time course of the EEGfrequency bands during SO were assessed in a group of 23 preadolescents(9–14 years, Experiment 1) and in a group of 36 older participants(59–81 years, Experiment 2). Additionally, we compared delta/beta ratio and delta activity during SO between these groups(Experiment 1: preadolescents, Experiment 2: elderly) and a group of40 young adults (18–29 years).Results: Experiment 1. Preadolescents showed a postSO increase(A) of power spectra in the low frequencies (0.5–6 Hz), with acentral predominance (0.5–2 Hz), (B) at 12–13 and 14–15 Hzlocalized over frontal and central areas, respectively, and (C) ofthe lowest beta over central areas. Preadolescents showed higherdelta/beta ratio in posterior areas (pre and postSO), higher deltapower over posterior (preSO) and centro-posterior areas (postSO)and reduced delta/beta ratio and delta power in frontal areas(postSO).Experiment 2. Elderly exhibited a power increase postSO of lower frequencies;the alpha band showed a particular pattern of postSO modifications;sigma power slightly increased postSO and its highest binsshowed a decrease in frontotemporal areas. Compared to youngadults, elderly displayed a reduced delta power and delta/beta ratioboth before and after SO.Conclusions: Preadolescents showed not entirely mature spindles anda more posterior delta activity, expression of strong homeostatic needfrom the “developing” areas; the decreased delta activity in elderlymight reflect a reduced homeostatic regulation during SO. Takentogether, these findings depict the scenario known for adults but withpeculiarities pointing to different homeostatic regulation likelyaccountable for the observed age-related SO dynamics.