How come desire and sati causes sleepness?
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.

https://dhammawheel.com/viewtopic.php?f=13&amp;t=35806&amp;p=