How come desire and sati causes sleepness?

I never felt asleep,When I am watching TV.But when I am studying or listening to *Dhamma*, **sometimes** I have felt asleep. What may be the cause of this ? But I can say, if I am falling asleep while watching TV,it is because of tiredness. How can we stop become sleepy by developing *Sati*? ( I read the advice from *Lord Buddha* to *Arahant Moggollana* to overcome sleepiness.But I need to know the cause of sleepiness and how to prevent it before it arises)