Gotta be honest, i never erg in the mornings anymore. Did do when i was super enthusiastic just to get on the erg during first few months of owning one... and never experienced any problems.
Now i do all my rowing in the late afternoons to early evenings... lots of good imo doing them at this time of the day. Your body is charged and has done lots of the normal daily activities (unless you sit behind an office desk all day of course)... i do plenty of walking during the day and stair climbing etc etc. Nicely supple and ready for stuff like the erg later in the day which is a great thing.
AM vs PM workouts
Re: AM vs PM workouts
46 yo male 5'10 88kg (Rowing since june 9th 2016) PB's 5k 19:22 30min 7518m