I used to have similar things.... I once slept while working seriously.....
I went through disciplinary procedure at work for that but said my problem,,,
I do not drink alcohol, sleep well, physically fit, do excercise too...
My manager suggested me to see my doctor....

The doctor send me for various test like blood sugar, thyroid, HB count.... all thing he could think of... and nothing came as a result.....
then he asked me.... how much water I drink... i said hardly any....
he told me to drink a lot of water... and I did
PROBLEM SOLVED !!!!