Training & Nutrition - HR way lower during morning training?

So this morning I felt guilty because I was too tired and lazy to get on the trainer last night and insteqad watched Zack & Miri make a porno. Funny movie BTW...
So this morning I got on my Fortius and did a quick 26km ride and noticed that my HR was almost 15 beats lower than it normally is when i have dome the same ride at night. I had not eaten nor had any coffee this morning. I usually train around 8pm or later and have geberally consumed at least one large Tim Horton coffee during the day. Today I could not go over about 158bpm (I'm taking it easy these days because I hurt my back...siatic (sp?) nerve) and usually I can get it up around 170-172bpm. Also i was at 135bpm for quite some time and had to push harder than normal to get it up in the 150's.
Is that normal? Anyone else have that happen?
Thanks
Chris

In my experience my HR doesn't change that much with time of day, but with being tired, having just eaten, caffiene, etc... Sounds like your HR is normally just high because of all the other stuff going on in the evening. It doesn't really matter what your HR is though; this is why HR is a relatively poor indicator for training...

Mine is typically lower in the morning. I would assume this is due to having rested the night before.
HR is better than nothing but power is the way to go. Now if I can ever convince myself to spend any $$.

Thanks for the responses.
I have not been able to get any real power training in for the last 2 weeks due to my back. I'm looking forward to being able to go hard soon! ...

My HR is usually lower in the morning. I have noticed that it's closely related to heat. On a hot afternoon my HR will be much higher than a cool afternoon or evening.

My HR is lower when it is cooler as well. It is very hard to maintain mine above 150 bpm when I'm riding in below-freezing temperatures. I can be working hard enough to breathe mostly through my mouth, and my HR still won't go that high.