Past perfect simple and continuous can normally be used interchangeably with verbs like work, wait, sleep, etc., but I think there have to exist some differences in meaning. I would like you please to help me find out the differences the following sentences may arise:
John found it difficult to wake up early this morning because he had worked hard the night before.
John found it difficult to wake up early this morning because he had been working hard the night before.
Both are grammatical, but had been working is more likely in this context, because the writer would probably want to emphasise the continuous nature of the hard work throughout the night.
Semantically OP's two sentences are identical, so it's really just a "stylistic choice".
But it's worth noting that Present Perfect Continuous normally implies a strong link between the past action and the current time of speaking (usually, the action continues up to the present).
By the same token, Past Perfect Continuous in OP's #2 more strongly links the "time further in the past" (when John worked) and the "reference time" (when he found it hard to wake up).
Personally, given the "reference time" is so close to the present (it's only this morning, not that morning, perhaps long ago), I wouldn't bother with either the perfect or the continuous aspects. John found it difficult to wake up early this morning because he worked hard last night.
First sentence : probably very hard, but possibly for a short time. Second sentence : suggest that all the night has been spent on hard work.
This has been asked and answered previously.
The differences between the two sentences are subtle at best.
There is an air of finality to "he had worked," whereas "he had been working" implies that John could also have been engaged in other pursuits besides working.
"He had been working" seems to go better with an additional thought. It is perhaps the more flexible of the two, being capable of being combined with another complete thought with a linking word. For example,
"John had been working hard last night, when in the middle of a project his phone rang and it was not good news. Needless to say, he found it difficult to wake up this morning."
"He had worked hard last night" could be combined with another sentence but seems to stand better on its own.
"John had worked hard last night. Consequently, he found it difficult to wake up this morning."
