I have been working 12hr night shifts for the past 12 months but recently I really struggle after about 8hrs into my shift I'm totally shattered until I almost blackout. Even If I stand in one place for a period of time I can feel my self going off balance almost collapsing with tiredness. If I sit in a chair I would be fast asleep in seconds and cant pull myself to wake. I'm 50 years old 5.7ft tall 69kg weight on no medications and have a good diet and believe reasonable sleep.