Thanks very much for your response.
The sleep mode kick in automatically and is no-brainer really if/when you regularly commute to and from work (as I started recently), and when device remains in the cradle throughout the day.
I take your point regarding the full shutdown but find it really unnecessary in the view of the above. Otherwise what's the point of this mode then?
When one takes infrequent strolls every now and then, or drives longer distances during holidays (as I did in the past) then that's another story, obviously, especially as I usually connect my satnav to my PC every now and then and get all trip's data downloaded to BaseCamp for posterity... Which actually fully shuts it down afterwards, hence me not experiencing this problem before.
Admittedly, sleep mode consumes battery a bit, and it wears with time, but then again: it's 7-year old device already, and I did replace its battery quite recently either.
So I take it it's the usual buggy firmware from Garmin that's the root cause then. I might give these guys a shout perhaps, when I have some time...