For security reasons, the Natural HR system will log you out if it has not been used for a specific time, by default this is 30 minutes. A 'Session timeout' message will appear, informing you that your session is going to expire and gives you the option to 'Stay logged in' or 'Logoff'.

To avoid this 'Session timeout' message, you simply need to click any link or refresh a page which will reset the timer. 

You can set the session timeout duration by going to 

Administration -> Company -> Settings -> Security settings

Enter the number of minutes, up to a maximum of 240, in the 'Session timeout duration' option. After clicking on 'Submit' to save your change, you will need to logout and back in for this to take effect