I totally get what you mean! Great question @margot 
Whether the sun touches it or not, it’s really a matter of personal preference.
This only matters if you believe it matters. You can simply gather your water when you wake up, regardless of the time.
The reason it’s worded like that is to discourage a beginner from just leaving the water there and then forgetting about it until a week later when they find it… 
Any blessing ritual requires to be mindful throughout, in this case throughout the night.
But the truth is that no recipe works for everyone. As you advance in your Craft, I encourage you to modify all and any recipes to suit your personal circumstances.
I really appreciate your feedback on the recipe. I will rethink and rewrite that bit! Thank you! 