First off regardless of a person being skinny or not, eating one meal a day with lots of coffee is not ideal. Your body needs certain nutrients and vitamins on a daily basis in order to function properly. I'm really not sure i understand what you're asking. Conserve energy for what?
Yeh, if you're not eating proper you need more sleep. And calorie restriction combined with excessive exercise could lead to fainting. But if you're still eating at least 1500 calories than less than three hours of moderate exercise shouldn't cause too much harm. Less than 1800 calories a day for prolonged periods though, isn't going to end well.
Chronometer.com can help you see all the nutrients you're getting. For the average person eating averagely 8-10 hours of sleep Is recommended. Everybody is different.
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