Water. Sometimes people feel very tired because they are dehydrated and they don't realize it.
A lack of vitamin B can also make you feel exhausted. So maybe if you don't have any health problems that can be negatively affected by particular vitamins, take a multivitamin which contains a good amount of vitamin B.
Also a lack of vegetables can make you super tired. Especially if you end up constipated because you don't eat enough veggies. Being constipated makes you very very tired.
Last but not least, don't eat junk food that fills your body with strange chemicals and processed fats that zap you of your strength and make you feel drowsy. Substitute all snacks with healthy snacks like an apple or a carrot or some boiled green beans, or other natural tasty things.
I hope these suggestions are helpful
Probably, a good sleep.