Because you're trying to lose weight -- not sleep.
Monitor your sleep as a part of your weight-loss regimen. Examine your own relationships between amount and quality of sleep and food choices, timing, and energy levels. Give yourself the best opportunity to do well in your attempt to lose weight, and recognize that sleep should not be ignored.
We don't know from this study whether diet is influencing sleep or sleep is influencing food choice, or both. But the evidence is abundant that these two pillars of health -- sleep and diet -- affect each other in a number of ways. We've seen a great deal of research that diet and weight control are strongly influenced by sleep, and that too little sleep can make eating healthfully more challenging.
Lay writings about soursop as a sleep aid are easy to find. In the West Indies, soursop leaves are commonly used as a sedative. In the Netherlands Antilles, the leaves are brewed to make a beverage that enhances sleep. The leaves can also be put into one's pillowcase to enhance sleep.
Here's the flipside to the story about foods that can help you sleep better: Eating a diet heavily composed of the "wrong" foods can interfere with your sleep.