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I've been facing this lately, where I will wake up in the middle of the night after sleeping for perhaps 2-3 hours and be wide awake, but when I go back to sleep, I wake up more tired than before.

Can anyone shed some light on this?

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Sleep duration certainly affects the quality of sleep, but it is nowhere near a simple correlation. It is just one of many factors, and these factors don't work in isolation. Statistically speaking, it's a multivariate model - and one which includes a differential equation, because the way sleep duration affects your sleep today depends on the sleep quality (and also the sleep duration) from yesterday. For example, if you go several day with too little sleep, but keeping firm going-to-bed and waking-up times, the ratio of your sleep phases changes significantly, giving you much more deep sleep. This is what compression therapy is based on.

And beyond that, the way you feel upon waking is also dependent on sleep quality in very complicated ways, and other factors (for example the synchronisation of the time at which you woke with your circadian cycle) play a much larger role than the sleep quality you got. Also, there are some unexpected effects, where getting too little sleep making you feel good - it's a kind of lightheaded, giddy state, somewhat comparable to having had a small glass of champagne.

So yes, there is a dependence, but you cannot expect that having had lots of sleep will make you feel good in the morning.

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My experience that it depends on it. You sleep in cycles, each cycle has about a 90 min duration. I think the quality depends on how many cycles you ended. It seems like you need to sleep more by an interruption.

Sleep was recorded on 10 nights in each of seven older subjects (mean age 55) and nine younger normal subjects (mean age 22). The duration of the NREM-REM (non-rapid eye movement—rapid eye movement) sleep cycles was determined, starting from the onset of NREM sleep.

In the older group an intra-individual comparison was made between the mean duration of cycles interrupted by one period of spontaneous wakefulness, within their NREM part, and the duration of intact cycles. The interrupted cycles were found to be on average 20 min longer than the intact ones, despite the mean duration of intervening wakefulness being only 10 min.

In the younger group an intra-individual comparison was made between the mean duration of intact cycles ranked as containing a large amount of stages 3+4 and the mean duration of intact cycles ranked low for stages 3+4. Similar comparisons were made in respect of the amount of stage REM, and also the summed amount of stages 3+4 and REM. The cycles containing higher amounts of stages 3+4, REM, and of 3+4+REM were found to be on average 9, 18 and 18 min longer.

The effect of cycle content on cycle duration was most manifest for the first cycle, and least for the third cycle of the night.

The results demonstrate that whereas there may be an average cycle duration of about 90 min individual cycles are very variable. The contents of a cycle influence its duration, and the onset of the REM period is not a strictly time-locked phenomenon.

If you wake up in the middle of a cycle or if you are interrupted often, then you will be tired. I think what happens by sleeping is much more complex, so this is just a short answer.

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From experience I cannot say quality is dependent on duration. For example, you can take a 30 minute power nap and feel much more refreshed than sleeping 8 hours. It may have something to do with dehydration and hunger during the long periods of sleep that disrupt our REM sleeping, our deep-sleep.

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