sleep apnea and other sleep-related breathing disorders, Narcolepsy
, Idiopathic hypersomnia
, Psychiatric disorders, Chronic use of drugs or alcohol, Other medical disorders, insufficient sleep, Sleep-wake cycle disorder circadian rhythm disorders (eg, nightshift work), etc.
You should seek medical help (sleep specialist) in helping you find out if there is any medical cause for excessive sleepiness.
In the mean time, be careful while doing work that could endanger you or others around you.