Is someone trying to hack into my SQL Server Database?
Looks like somebody is really trying to hack into your SQL Server with brute force attacks. I recommend taking a look at this Whitepaper on SQL Server security, that will help you a lot.

Since most attacks happen at the same time, make sure you don't have a security scanner on your network that runs its checks at that time. Products such as Nessus can perform security scans and simulate brute force attacks on your servers in order to help you fix the security issues they find.