Baby moves when u lie to sleep?

when i lie down or sit watching tv i dont feel my baby moving,but as soon as i lie to sleep at nite my baby starts knocking at me i.e moving; my ques is why babies move during our sleep time or when we rest.How can i feel more of his/her movements.have heard drink cold water and lie down to feel movements.i m 21 weeks pregnant.is it imp to feel your baby move daily .pls ans