When should I measure my height, and which is the 'true' result?

[BoBSirDylan]

Last night I measured my height and I was just over 5"9, so I thought that was my height. This morning I measured myself to see if I grew and I was 5"10! I thought it was impossible to grow that much in one night, so I asked my Mum and she said that it's to do with the gravity, as in the morning when you wake up you have had no gravity acting verticaly down your spine for hours so it decompresses, but later in the evening because of the gravity acting on downards and compressing your spine you are apparently shorter.

Anyway... which one is my 'true' height then! Which is the height I should be telling people I am? 5"9 or 5"10? Because it's a big difference! 5"9 is below average and 5"10 is above! Aha!
So yeah, does anyone know which one is my true height? And also does anyone know the 'correct' or 'best' time to measure you'rself??
thanks guys/girls