Sometimes during the day i try to sleep or laying still, ill suddenly jerk rather hard. also my heart rate increases before going to sleep. causes?

Hypnagogic jerk: You are not a Jerk! But the sudden muscle twitch that you describe is very common, and is called a hypnagogic jerk. There is a theory that this is a leftover reflex; a primate falling asleep whose muscles relax too much might fall out of a tree, and so we wake up to grab a limb. With that startling jump, the heart rate may accelerate. Whatever the cause, it is a normal part of falling asleep.