IMPO, by doing a passive activity, the brain switches to a different wavelength. if i remember correctly, in different levels of consciousness there is a co-relation to the brain activity level. so if i am doing something that i need to respond to, my brain is more active as responding to a post requires a higher level of brain activity.
as for watching TV, i do not need to respond like i would to a post. so in the long run the brain 'winds down', & some just drifts into sleep.