why does j thinks he cant get back to sleep after he has a rough night near picnic point?

From the text;

"I WOKE at six the next morning; and found George awake too. We both turned round, and tried to go to sleep again, but we could not. Had there been any particular reason why we should not have gone to sleep again, but have got up and dressed then and there, we should have dropped off while we were looking at our watches, and have slept till ten. As there was no earthly necessity for our getting up under another two hours at the very least, and our getting up at that time was an utter absurdity, it was only in keeping with the natural cussedness of things in general that we should both feel that lying down for five minutes more would be death to us."

....... there was no particular reason!