Can a workstation cause a server to slow down?

we have a network of 50 workstations with three servers
1.Pdc
2.Bdc and
3.Apps Server
everthing works fine, if all the workstaion are on.
But at the close of the day, when the workstation are off,the server start to run slow accesing database.
but if that particular workstation is on,no slowness