system
Given the following post, respond as if you're commenting on Reddit:
human
What are the physical consequences of skipping breakfast, and why is it so bad?

As the title says, it beeing considered the most important meal of the day, what happens on a biological level and how does that impact the person throughout the day? Like affecting someone's mood and energy, so on. I pull some crazy hours sometime, going to sleep at late night and waking up almost by the end of the morning, so plenty of times, lunch is my breakfast wich I take it isn't very healthy as well.
gpt
Your body relies on your liver for glucose stores when you don't eat. Realistically, a healthy liver contains about 12-16 hours of glucose in it that your body can use during fast- some sources put it closer to 16, some closer to 12. However, after that, your body relies on a process called gluconeogenesis, where your body produces the glucose it needs to supply the brain's and red blood cells' glucose needs. 

What does your body break down to make glucose, during gluconeogenesis? The majority of it is amino acids, taken from breaking down your body's muscle (about 60%), and the rest (about 30%) comes from body fat, lactate, and pyruvate from your muscles.

So, the consequences of skipping breakfast and fasting more than 12 hours include: using up your body's glucose reserve and starting up gluconeogenesis, which largely relies on muscle. This isn't so great, since you want your body to to keep muscle; plus gluconeogenesis produces much less glucose than you need to feel perky (it's just trying to keep your brain and RBCs alive) so you feel tired, have less energy to do work, etc. 

When you eat breakfast, your body will use that for energy, plus restock your liver for the next night of fasting. Eat breakfast! :)

Edit: this does not mean that with no breakfast, your body is going to start eating itself from the inside out! It simply means that your body is using muscle-derived amino acids as a substrate for gluconeogenesis. You're not going to wake up one day after skipping breakfast for a year and have no muscles left! :) It's simply healthier to have your body use glucose you just ate, rather than go into gluconeogenesis, especially for hormonal reasons (see other comments below).