Yes, that’s true! And at least for me it is surprisingly accurate with 1h30. I started the habit of looking at the clock everytime I fall to sleep and wake up and it is always after 4.5, 6, 7.5 or 9 hours of sleep!
Also I heard that it is better to sleep 7.5 hours compared to 8 hours, because being waken up from deep sleep allegedly will make you more tired than these 30 minutes of extra sleep.
The entire text I misread soldering iron for a regular iron. Up until the very last paragraph. Now everything makes a lot more sense.