Most system processes in LotWorks run overnight to cut down on system resources, and to improve performance. These include scheduled notifications, memos, reports, etc.
Lot Holds, and other system processes that deal specifically with EXPIRY times, occur at the exact time they were scheduled to occur.
This means that a lot hold requested at 8am will expire at 8am, X days later (X being the hold period length). Depending on your developer, this hold period can be anywhere from 7-28 days long.