Knee tightness/numbness

So, for the past few months or so I've been waking up throughout the night (not every night though), with my knee feeling tight and numb, as if it's going to lock up, so I'll rub it and try moving it, like kicking or walking to try and get it to stop. I also move a lot throughout the night, so, I know it's not just "falling asleep". I feel it twitch sometimes throughout the day. I'm not sure what it is or why it's doing it?? I tried googling it but nothing really comes up.. It's not painful really, like I don't have to take pain killers or use a heating pad. Anyone have any suggestions? Could he be from dehydration/not drinking enough water?