Why is SQL Server 2008 R2 Management Studio making me login again?
Possible simple reasons:


Connection is broken between your PC and SQL Server (for example some Switch can be powered-down, re-started etc)
Connection can be closed from SQL-Server side if your admins make some kind of maintenance and simply kill connections, or if your Server is running in Virtualized envronment, it's instance could be "paused" to make a snapshot.


list of reasons can be really big



you can start analysing using simple tool: when you go home, just run:

ping -t NAMEOFYOURSERVER &gt;c:\temp\log.txt


this will start permanent ping and when you will come next day - just Ctrl-Break and look into the file if there are Time-Outs. This way you will know if at least IP connection was alive.