You can change your status message on Whatsapp to whatever you like. All other users can see this message. This way you can show your contacts what you are doing (Sleeping, working, sporting etc). You can also see which users have started the application and are online. If they are not online the last moment they were online is shown.
How does this work?
iPhone or Blackberry: Go to the status screen. Select status from the list or add a new status by clicking on the plus (+) sign on the right top. On the iPhone you can change any status by clicking on the edit button on the top left.
Android: Go to chats, choose the menu button and select status. Choose a status from the list of preset types of status.
Nokia S40 / S60: Go to the status screen. Choose a status from the list of preset types of status. You can add new types of status through Options -> New Status.
Windows Phone: Find the three dots “…” on the bottom right of your screen. Drag them up and choose “my status”. Tap the top textfield with your current status and change it to whatever you like. When you are done press the checkmark.
Note that it can take a while for Whatsapp to update the status to all your friends.