On the limited (Cisco) VPN systems that I've used, the VPN connectivity is broken when my computer is put to sleep and must be re-established on wakeup. Is this a necessary property of VPN?
I think Bob Somers hit the answer, but there's another possibility. You could be sleeping your computer through the rekey interval, which breaks the connection.
VPNs negotiate a session key for every connection. This key gets "stale" after a while, and may be vulnerable to attack if enough data has been passed with it. To maintain the security of the VPN channel, there is a rekey interval where both ends automatically renegotiate a fresh session key. This is usually transparent to both sides of the tunnel, assuming both sides are connected. When you put your computer to sleep, it's possible the rekey interval is up during that time, and the VPN host closes the connection because it can't renegotiate the session key.