After Installing El Capitan I Will be Logged Out Frequently
From 
https://support.apple.com/kb/PH21782:


  OS X El Capitan: Set your Mac to log out when not in use
  
  You can set your Mac to automatically log out the current user after a
  set period of inactivity. This helps ensure that your information is
  secure when your Mac is unattended.
  
  
  Choose Apple menu > System Preferences, click Security &amp; Privacy, then click General.
  Click the lock icon to unlock it, then enter an administrator name and password.
  Click Advanced.
  Select “Log out after... minutes of inactivity.”
  Set the amount of time before the user is automatically logged out.