You are dealing with data caching. In particular the limited scope of data caching. In essence what you are seeing is that data gets aged out of your data cache and needs to be reloaded. Once the data is in the cache, the access to it is simply a call to a memory structure instead of the much more expensive disk access. My best guess without any more information is that you have processes that run during the night that essentially utilize...
all or almost all of your data cache so that the first test during the day requires disk access for data since it doesn't exist in the data cache at that point.
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