I very often feel tired at ard 4 to 6 in the afternn - headache, sleepy, lack focus, but aftr dinner it's fine. how to improve? anything worrying?

Afternoon fatigue: You may be hypoglycemic- low sugar. consider a protein/ carb snack, like 1/2 a sandwich at the time and see if it improve your state. You could also check your blood sugar at the time. Follow up with a doctor

Sleep: You may not be getting enough sleep at night trying to make sure that you go to bed and get at least 8 to 10 hours sleep at night otherwise I cannot think of anything else that would be a problem

Need more details: More details need to be obtained - what time do you arise in the AM? What are your sleep habits? Are you eating rituals regular with good nutritional choices, or do you grab a latte for breakfast and a Diet Coke for lunch? Do you use your electronic devices right before going to bed at night? Do you ever collapse? Please see your doctor for a careful assessment of your fatigue.