Can someone please explain to me how bbt works?

So this is my graph so far. I had to start BBT on cycle day 5 as that’s when my thermometer decided to arrive. So my Son has had me up in the night and I’ve always done it around 4am (give or take 30 minutes) after 3 hours of consecutive sleep.

Can someone please explain to me as the days go on, what sort of line I should be seeing on the run up to ovulation? Also, for the 3 days, my readings have been 36.45, 36.26 and 36.20. Is it normal for temperature to be going down like this at this stage? I’ve just finished my period, so a day after finishing that.

Any help would be appreciated!!