Are you alert and restless at 4 am? Then you can say you're wakeful, or not able to sleep at all. You might as well get up and start your day.
If you're wide awake, especially when you should be asleep, you're wakeful. A wakeful baby can be perfectly pleasant in the middle of the day but is a lot less fun after midnight. Many new parents spend weeks or months of wakeful nights, and even getting a new puppy can lead to a long, wakeful period of your life. Before it meant simply "marked by being awake," this adjective meant "diligent."