From DD-WRT Wiki
 What it can be used for
Scheduled reboot is often used to give the routers a little clearing now and then to keep its performance at its peak. It can also be used in combination with uPnP with the setting "clear ports at startup" enabled to give the benefits of uPnP without the drawbacks of gaps in your security (for long periods of time anyway).
I find that using a scheduled reboot often gives a brilliant transfer and receive rate for wireless which makes a huge difference when in client mode. The higher to the maximum transfer and receive (whilst assuming no other wireless clients are heavily using it such as downloading) I can receive extremely low ping between my primary router and secondary router - around 1-2 ms average with a distance of around 50 metres set in the ACK timer.
 Not working as intended
On DD-WRT v24-sp2 (06/14/11) Build 17201 - I have found that in client mode, scheduled reboot does not work or more so, it only does it for the first time set, despite it being put as "every day." To fix this I used code which a moderator passed onto me and hopefully it works for you too.
- Go to DD-WRT configuration page
- Disable scheduled reboot in Administration -> Keep Alive
- Administration -> Commands
echo "startservice run_rc_shutdown; /sbin/reboot" > /tmp/restart_router chmod a+x /tmp/restart_router echo "55 5 * * * root /tmp/restart_router" > /tmp/cron.d/restartrouter
- Save startup
- You can edit this at any time
- I would also recommend giving your router a reboot just to be sure. Administration -> Management should be good enough
Editors note: I have no idea how to fix the numbers whilst giving the code box.