Sleepy Central Locking

I have a '54 plate TF and the central locking does something a bit weird. If I lock it and then unlock again within a minute or so, I get good range (10-15 feet). However once it's been locked for more than 5 mins, it seems to go to sleep and will only unlock when I'm right next to the car. Does the system go into some sort of power down mode after a certain amount of time which decreases the range? It's not a major issue but I'm interested to figure out why it does it.

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