The military thinks that 22 is about optium for members. You need to be between 18.1-24.9 but they would rather you be at 22.
I think it depends on the individual but within reason. I have a BMI of around 20; I feel sluggish and tired when I put on a few pounds. My muscles and joints are used to carrying around a lighter weight.
There is also a difference between naturally having a BMI of 18-19 because of your metabolism and forcing yourself down to that BMI by dieting.
I also don't think the time of day that you eat matters very much as your body is efficient and will store the calories (energy) from the food that you don't use right away so that you can use the calories when you need them. So, if you have a light evening meal and a big breakfast, your body will use the calories from your breakfast for whatever you need to do that morning.But if you have a big evening meal and a light breakfast (or skip breakfast), your body will store the calories from your evening meal overnight and use that to get you going in the morning. What really matters is the total amount of calories you are getting.
Also, you have a mechanism in your liver for making sure your blood sugar doens't get too low. It pumps sugar into your blood as your activity level dictates. Unles you have diabetes, have other medical problems related to blood sugar, or have liver problems, your blood sugar will not get too low even if you haven't eaten for a while, assuming that your overall intake of carbs is healthy. You will always have enough sugar in your blood for your body to function properly.