Why Do I Feel Dull And Have Less Concentration In The Mornings ?

am 52 years old.with no health problems and no kids of my own, working in a busy childcare centre in Australia which can get hectic for me at this age. I do love working with children but when this kind of thing happens to me I get worried. The problem is in the mornings I dont feel alert like i feel kinda dull in my head . cant concentrate in my work..feel lazy too and loose interest and lifeless with no energy, feel sad too. But what I noticed after my lunch i feel better..kinda of alert and woken up from my dullness. This has been happening to me since many years off and on. like once in six months or so. it last for 4 to 5 days. Everytime I go to the doctors..they advise me for a blood check up and all goes well.. Now again on the 18th i.e. Friday this kind of thing happened to me again..So i have taken leave from Wednesday since I dont feel good about this as I cant perform well at work.and feel better at home rested..but no interest to even cook. but now I am beginning to get well. But I am afraid i might get this problem again coz re-occurs after some months.