What is the difference between these sentences using past perfect tense?

Question

The second sentence is said by using had. This sounds like past perfect tense. What is the difference if I don't use had?

I slept less often when I was in Canada.

I had slept less often when I was in Canada.

Answer 1

"I slept less often when I was in Canada" just states the fact about how often the speaker slept while in Canada.

"I had slept less often when I was in Canada" states the same fact but also suggests that after leaving Canada, the speaker slept more often than when in Canada.

Explanation

Here's why: the perfect tense means that an action is complete at the time being described.

There are many reasons why you might want to suggest that an action is complete, there are many things that "completion" might mean, and the most relevant action completed might not even be named by the verb. The specifics are provided by context, which can greatly change the meaning of the perfect tense.

One common reason to use a perfect tense is to contrast with what came after the action was completed. This reason leads to the interpretation above: "my period of sleeping-less-often was completed while I was in Canada; it did not continue afterward."

Another common reason is to indicate that the completed state of a process produced some relevant effect at the time being described:

After years of grogginess whenever venturing north of the 49th parallel, my experiments with the Mounties' amphetamines finally succeeded. I had slept less often while in Canada. Indeed, I have slept less often ever since.

This time, the second sentence means "my period of trying-to-sleep-less-often-while-in-Canada successfully completed" or really, "I slept less often because I was under the influence of the amphetamine experiments which had successfully completed at that time." In the third sentence, the present perfect means that the completed action (the experimentation or the attempts at sleeping less often, take your pick) continued to exert its effect after the speaker's time in Canada, continuing to the present day.

Answer 2

The perfect aspect is often used in sentences taking place over a span of time. For this reason, it's important to analyse its use in context. The second sentence, as it stands,

I had slept less often when I was in Canada.

is begging for more context, for example:

I told my English GP that I had slept less often when I was in Canada.

Now, the time span is clearer and the sentence sounds neater.

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Let's consider now the first sentence and the second sentence with the extended context:

I slept less often when I was in Canada.

I told my English GP that I had slept less often when I was in Canada.

The difference due to using the perfect aspect is that, although both sentences imply that your sleeping patterns have now returned to normality, the latter sentence highlights the span of time when this took place.

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Please, refer to this canonical post for a more thorough description.

Answer 3

I think this should all be covered by Canonical Post #2: What is the perfect, and how should I use it?, but that's a pretty long answer. The relevant bit for OP here is...

Use perfect constructions to introduce prior eventualities as context for the current discussion.

What that actually means is if you're currently discussing something in the present, and you want to refer to something that happened earlier, you use simple past. But if you're discussing something that's already in the past, you might use past perfect to refer to something even earlier. To illustrate...

1: "I don't know why I am so tired all the time. I slept less often when I was in Canada, and I wasn't tired then."

2: "I didn't know why I was so tired all the time. I had slept less often when I was in Canada, and I wasn't tired then."

Note that many pedants would say it should be ...I hadn't been tired then in #2, but in practice many if not most native speakers wouldn't bother with that (personally, I wouldn't necessarily bother with past perfect for the preceding had slept either, since the fact that you're talking about a time even further in the past is obvious anyway.