There a host of possible reasons.
One of the first things to do is to get what is called an hbA1c test, which is used for diabetic screening. High glucose levels are the most common reason for this kind of sleepiness.
A work-up by a speciality called an endochronologist is also in order.
Another possibility is a disturbance of what is called the "circadian clock" which determines sleep cycles.
Daily exposure to sunlight for 45 minutes at the same time often straightens this out. Do a google search on "circadian rhythym".
There is also a genetic componant. There is some survival value in being about to "turn oneself off". Some people do so, and it has been theorized they conserve energy in this manner. The fact you have two jobs suggests that you are a hard-working conscientious individual and your body wants to optimize "rest" time.
You might need to be evaluated for narcolepsy. I have suffered from similar troubles over the last year. I take a stimulant called modafinil which has been a lifesaver. I could fall asleep at the dinner table, leaning against a wall, and even once on the phone. Now I'm not speeded up or wired in any way, just able to control when and where I sleep.
Excellent point helen41 and a good drug suggestion.