Prolonged sedentary behavior is linked to poorer metabolic health, yet its effect on cognitive load and brain function remains unclear. Evidence indicates that "mentally active" sedentary tasks (e.g., reading) may support cognition, whereas "mentally passive" tasks (e.g., scrolling social media) may impair it. The cognitive demands associated with these behaviors across the general population are still poorly defined. This project will compare the acute neurophysiological and perceptual responses elicited by mentally active versus passive sedentary tasks across various age groups and in individuals with specific health conditions (i.e., obesity), clarifying how these behaviors differ in the cognitive load they impose. Neuro-cognitive, physiological, and perceptual responses will be assessed with a multimodal battery that includes portable electroencephalography combined with functional near-infrared spectroscopy (EEG + fNIRS; MUSE), eye-tracking (Pupil Core), alertness and visual fatigue via critical flicker fusion testing (CFFT; Lafayette Instrument), autonomic balance through heart-rate variability (HRV) recorded with a Polar H10 monitor, and the self-reported cognitive load assessed using the NASA Task Load Index (NASA-TLX). We hypothesise that mentally passive sedentary activities will elicit a lower cognitive load than mentally active tasks. By comparing different age groups and health conditions within a single protocol, the study will generate an initial set of group-specific data; subsequent independent studies can build on these findings to explore moderation effects in greater depth. Collectively, the results will provide both the theoretical rationale and the empirical evidence needed to sustain the "mentally active" versus "mentally passive" terminology in sedentary-behaviour research, with the ultimate aim of improving mental and cognitive health.

Inclusion Criteria:

• Children aged 5 to 17 years, attending school.
• Adults aged 18 and above, without an upper age limit.
• Participants with and without specific health conditions, such as obesity.
• Ability to perform sedentary activities and comprehend instructions.

Exclusion Criteria:

• Severe visual impairments affecting reading or text visualization.
• Use of conventional glasses and history of migraine or epilepsy (due to preventive request for the flicker fusion test)
• Current use of medications that significantly influence cognitive function (unless the health condition being evaluated requires it).
• Other conditions interfering with cognitive or neurophysiological assessments.

Doble-blind. Both participants and the statistician were blinded to the intervention condition; however, the assessors administering the activities were not blinded.