I read somewhere at some point about using the difference between resting morning and standing heart rates to assess fatigue level. Today, I finally found out that it's called Orthostatic Heart Rate, and also found a nice little article explaining it.
I use the following protocol:
Upon waking up, remain lying down, take my resting pulse for 30 seconds. Stand up using the minimum energy required and take my standing pulse for another 30 seconds. The difference between these will be my orthostatic rate.