Romberg test... what does it mean?

While in hospital and wondering how to tackle the neurologist who was coming to see me, I learned about the Romberg Test. I was chuffed that I have a test that I can perform myself that perfectly and easily demonstrates that my symptoms are physical.

I have been trying it on and off because I've been second-guessing myself about whether my symptoms are actually real (it's that old adage, repeat something enough and eventually a person will believe you). Every time I do it I wobble over despite my best efforts.

However, first thing this morning I was able to balance with 2 feet together perfectly for ages. I was surprised.
I did it again this evening and wobbled right over immediately.

So here's my question (yes, finally, I got to it!):
Why do I pass my Romberg Test in the morning and not a few hours later in the day?

Rachel xx