Its not a case of the Mondays, hehe
Here is a list of reasons for why you probably feel tired today.
1.) You Don’t Drink Enough Fluid /Water
This is so common. Men need to drink about 13 cups per day and women 9 cups per day. All drinks count with the exception of alcohol. Yes you can include your caffeinated beverages too.
2.) You Backlog Your Calories /Calorie Timing
When people try to diet or have no idea how much food they need each day they tend to consume very little in the morning and then overload at dinner. This is a terrible strategy. It doesn’t work for weight loss because you’ll end up binge eating or over eating and you’ll feel tired during the day. Eat your calories when you need them to help you fuel your most productive times of the day.
3.) Lack of 
Sleep
Chronic sleep loss will make you feel tired and grouchy and effect brain functioning. Most people need about 7 to 9 hours of sleep per day and athletes need about 9 to 10. When you sleep more you will function better and probably get more done. Plus you’ll feel a lot better.
— M.Spano
Plan Act Win 