Working 18hours a day any tips on staying alert safely?

I worked 12am to 6.30am then went straight to my other job and will finish at 5pm this arvo. I can hardly keep my eyes open! i have to do it again and i will only get 5hours sleep before this. I want to no if anyone has any good methods on staying awake. Ive tried coffee, energy drinks. I dont want to use anything illegal like drugs or any of those NO DOZE tablets coz i hate them! Can anyone help me out here :-(