I've had this a few times now and it's a little worrying. First I feel a steadily building pins and needles sensation, usually always turning my left hand numb. First it starts as a weird sensation, before steadily numbing my hand, to the point only the barest sensation exists, where I can't even bite there or have trouble tying. Then this "area" of numbness spreads. My hand slowly gets better, but then I feel it spread to my cheek and mouth, where it gets numb to the point that it takes a while before I even notice chewing on my lip. Then it once again switches down from my cheek to my right arm, where it once again slowly goes numb (this is also when I have the most trouble typing since I'm right handed.). Eventually, even this sensation slowly dissapears. This has happened about 5-10 times in my life, all over the last few years.