Antonio,
the default time for your map services to recycle is at midnight (00:00). We see the same activity on our servers at midnight. All of the map services are essentially stopping and restarting. Here are the settings in ArcGIS Server Manager:
Midnight is typically a slow time for our servers so there is no real impact on our users. You can change the default time for the recycle so not all your services are offline at the same time.
Bernie.