This is not a good use of therefore, since there is no clear causal connection between the statements that precede and follow it - the statements are related, but not causally. The first two sentences simply state the amount of time needed to get a good night's sleep. The third sentence describes what happens if you don't get enough sleep, but that isn't causally related to the specific amount of time that makes a good night's sleep.
The paragraph as written could be paraphrased as follows:
A chronic lack of sleep impacts your ability to pay attention because
most people need 7-8 hours of uninterrupted sleep.
This sentence isn't logically true, the effects of chronic lack of sleep don't occur because people need 7-8 hours of sleep. They occur because people don't get enough sleep, however much that is - the effects of chronic lack of sleep is not a consequence of the amount of time people need to sleep. Chronic lack of sleep would still have the same effects even if people needed 15-16 hours of sleep per day. It is not a logical consequence of the fact that people go through 4-6 cycles of 90 minutes each.
To think of it another way, if you told an alien that humans need 7-8 hours of sleep, they would have no means of concluding from that statement alone that a chronic lack of sleep has all the effects you listed. Conversely, if you told them the effects of chronic lack of sleep, they would have no means of deducing how much sleep humans need.
