Make it free!!
I am not paying £13.99 for an app to tell me when I will be tired!!
I can look at my roster and see a Tokyo 4 day trip followed by 2 days off then a 3 day trip to the states I know I am going to be tired, no app in the world will tell me anything I won't already know!
The only way to 100% beat jetlag is not to fly, and I have bills to pay!