Most often this is a router/connection problem or interference from Bluetooth/gaming devices. Turning the router off and back on may correct the problem if it is router related or reloading the router firmware (save your settings/configuration first) may fix the problem.
The first place I would check as searay suggested is the router logs. This would assist with ruling out that it is related to the internet connection itself.
Are wired LAN connections fine during the time the WiFi connections fail?
Do all your WiFi devices disconnect at once? When this happens is it constant or just an occasional disconnect of one device now and then?
If you know your neighbors you might check to see if they are experiencing similar problems around the same time of day. If they are it is probably interference in the area.
As long as you have a strong WPA2 WiFi password (use upper/lowercase letters and at least one number and special character) I would not worry about the security of your system. The longer the password the better and the more crazy the password the better.
If it is interference, then determining what/where it is originating from can be a challenge.
There is a program called inSSIDer which is great for seeing network activity (routers) in your immediate area (
http://www.metageek.net/products/inssider/). This however will not show interference from Bluetooth and gaming devices. To see this type of interference requires some type of spectrum analyzer which can be expensive and likely not worth the money.
Hope this helps.
