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 reflect mutually-exclusive states, but instead represent local phenomena. Despite the massive age-related modifications occurring during lifespan, the electrophysiological (EEG) Sleep Onset (SO) features in preadolescence and healthy aging have not been exhaustively investigated. Thus, we aimed to describe spatiotemporal EEG dynamics of SO in preadolescents and older adults. Methods: The pre- vs- post-SO changes in the topography of EEG power (1-Hz-frequency-resolution) and the time course of the EEG frequency 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 of 40 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 a central predominance (0.5–2 Hz), (B) at 12–13 and 14–15 Hz localized over frontal and central areas, respectively, and (C) of the lowest beta over central areas. Preadolescents showed higher delta/beta ratio in posterior areas (pre and postSO), higher delta power 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 bins showed a decrease in frontotemporal areas. Compared to young adults, elderly displayed a reduced delta power and delta/beta ratio both before and after SO. Conclusions: Preadolescents showed not entirely mature spindles and a more posterior delta activity, expression of strong homeostatic need from the “developing” areas; the decreased delta activity in elderly might reflect a reduced homeostatic regulation during SO. Taken together, these findings depict the scenario known for adults but with peculiarities pointing to different homeostatic regulation likely accountable for the observed age-related SO dynamics.