There is a lot of concern at the moment that WiFi signals could be having an impact on our health. We have not been using WiFi for all that long, compared to how long some other 'health risks' have been around - and considering there was once a time when shoe stores used X-Rays to determine whether shoes fit properly, it is reasonable to worry that WiFi could be the next "oh, oops, that was dangerous" scandal. It's unlikely that will happen though. WiFi waves are not very intense or strong, they are transmitted over long distances, and they are essentially the same wavelength as cosmic background radiation. If you're nervous about WiFi you may as well be nervous about going outside.
Your WiFi router, if left in a sensible place such as on top of a bookcase in your living room or study, is not going to expose you to masses of radiation - you'll have greater exposure from your cell phone over a long phone call than you would an entire year of just 'using your laptop'. This means that you can be reasonably confident that your WiFi connection is safe. According to the World Health Organisation, there have been 25,000 articles published over the last 30 years relating to in-depth study of the effects of WiFi on adults. There are some gaps in the knowledge of how prolonged cell phone exposure could impact children, but the body of knowledge about the impact on children is extensive and WiFi is generally regarded to be safe. If you're concerned, then, by all means, turn off your router when it's not in use, but remember that WiFi is ubiquitous even outside now. There are other things that are a clearer health risk such as being sedentary, that should be addressed for most people.