So tired

I'm facing a long journey on my own with the kids.  I'm a little nervous, but there are a couple of red bulls in the car in case I get tired.  Daniel picked one up, and brought it into the house.  He was chatting about it is to stop you being tired, and that daddy likes them.  I took the can back to the car, and came back to find a very sad little boy.  I wasn't sure what had upset him in the few seconds I had been outside, until he said through sobs "I'm so tired".