I think it's no problem and would also suggest dateorade and possibly soaking the dates the night before. Happy Ramadan :)
Also did you see this post?
Citrus stored in a closed container might keep well, although it might need a good shaking at the time of drinking. I am not sure about the water melon though. Never juiced one myself:D
Some people have had good results storing datorade for a few hours and will consume it at work for lunch. I am not sure if the bananas will oxidize or not, but even if they do, as long as they are not fermented, it might be ok.
Another idea might be to just cut the watermelon into big bite sized pieces before sleeping and eating it verses juices and smoothies.
If all else fails, ripe bananas are the easiest thing to eat at that time fresh from the peel. No bleary eyed prepping:D
Thanks for sharing with us your experiences. I was a bit curious about the water melon.