Shields

I have learnt what the shields do but i would some guidance as to how they work.
Some Mornings when i boot the counters tick up very fast other Mornings they hardly tick up at all and sometimes never tick up
I do the same things every Morning.Like Check emails, check updates,read the news etc,etc and visit the Forum,listen to ClassicFM
Why should the shields(counters) vary so much?
Please could i have some advice.
Thanks.