Why are evenings suddenly hard for my dog?

I have a Labrador who’s now four years old. He’s usually happy to doze or snuggle whenever we’re not outside. He gets at least a couple hours of exercise per day, and he used to be satisfied with that.

For the last week or so, he’s been acting weird at night. He grabs toys to pounce on and destroy. He whines at me and wants to play. When we do play (never in response to the whining), he gets too wild and we need to stop fairly soon. That, also, is new. He’s always active, but he usually plays gently enough with me.

He does this intermittently for a couple hours and stops when I go to bed. It’s like he’s waiting for me to settle down so he can go to sleep, too.

It might be important to note that one of his “girlfriends” is in heat, and he’s an intact male. However, why does he only seem to suffer in the evenings?

As I write this, he’s curled up and whining very quietly. It’s sad to watch. I’d like to help him.