Working alone in a retail store?

[sayre]

I have been workin the midnight shift for about 4 yrs as a night manager. we close at 1 but have deliveries all night so i have to stay all night. I work in a retail grocery store which is in the union. at 1 am i let the cashier out and walk her to the car and go back into work and from 1am till 6 am i am completley by my self. they have been doing this for 4yrs i have been telling them how unsafe i feel and they keep telling me that they cant find anyone to work the midnight shift with me which is ridiculous someone out there will. but are they aloud to do this i am required to lift up to 70lbs and stock shelves all by myself. shouldnt there be some saftey law about this. if i call my union im afraid ill be demoted and loose my full time and insurance. is there anything that can be done i am a 100lb 23yr old female who cant take on someone if i get robbed after walking the last worker to her car. i also worked till 5am alone and when i got off work i went into labor only an hr later i could have went into labor there with no help