water pressure on gas boiler?

The Blue pressure needle on my gas boiler should be reading about 1.5 . However, when the system is set on low during the night i.e. minimum heat output whilst in bed the needle drops down to zero but once the heating comes on again about 7-00am the needle goes up to as high as 2.5 . Why does this occur and should the needle stay at 1.5 even when heating is on low at night .

Thank you