Apparently the Lighting Research Center has found proof that working in the evening with light from a screen, whether it is a tablet or a laptop, is bad for sleeping. The reason for this is that it inhibits the production of Melatonin, which has a negative effect on the day and night rhythm. The distance and the amount of light have an effect on the production loss. The farther away the screen is and the dimmer the screen is both have a positive effect on the melatonin production. I would say that a laptop is for that reason better than a tablet, because of the distance. One of the commenters on Tabletsmagazine is using Flux on iOS devices. The nice thing about flux is that it not only dims the screen, but also changes light colours to a more yellowish variant. iOS devices have to be jailbroken though. But far most the best thing of course is read on the Kindle, or any other e-ink screen for the win. I knew already that I can not read for more than half an hour on the iPad because of the backlight. My eyes get tired and can not focus anymore. On the Kindle I can read for hours. And now it not only proofs that it is a better experience, but also that it has a positive effect on your sleep pattern.
Although I somehow see a problem in this reasoning, my own reasoning. With the Kindle I can read for hours, and it sometimes happens that I can not stop reading and have to stop early in the morning for a short sleep. That will not happen with the iPad, for many more reasons, including the weight.