If conditions have improved then I’d not expect the line to resync on its own so I assume you mean that if you force a reconnect, it does so at a better speed? On that assumption a possible scenario might be as follows....
Firstly, suppose you have neighbours who are less concerned with connection speeds than we, on this forum, are. They leave everything alone. At this time of year, with longer hours of darkness, owing to well understood variations propagation of radio waves in darkness, their own connections may experience higher error rates. If these error rates are bad enough, your neighbours’ connections may drop and reconnect in middle of the night. That will happen, by definition, during hours of darkness so by competing with the interference, they will reconnect at a poorer speed.
Subsequently, if you then force a reconnect in daylight hours, you will negotiate a speed that is not impacted by the interference that goes with darkness. But better still, because your neighbours’ speeds have dropped there will be less cross talk, so your speed will be even higher than its usual ‘daylight’ value.
Unfortunately you can’t beat natural phenomena so if above is the explanation I’d expect the faster connection to be all the more fragile in hours of darkness and itself perhaps, to reconnect in middle of the night. If that happens regularly, DLM may start to intervene and then, your speed might be worse than ever.