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)
Sloth and Topher is the fifth Mara.
It activated when all other previous four maras are inactivated.
For instance, when you watch TV the first mara is in charge.
When you listen to Dhamma if the fourth Mara is not in charge you will go to sleep.
This is a complex case.
The way to avoid this is to practice walking meditation.

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