Server is Down in the morning

Aside from a posible UPS battery being bad,
what could cause my 4.11 server to shut down
after hours. It has to be happening around
4 am or so because the back-up operation
is completing.

I have come in and found the server was off.
Boots normally, no errors msgs displayed.

I’d really like to get this solved because the VP is using it (the server “failure”)
as grounds for leveraging in a Win2k server instead of upgrading from NetWare 4.11 to
5.1.

Thanks.

Max