Lena's headache didn't improve after she had slept, in fact it felt worse the next day and she had too much to do which only made her more annoyed.
The only positive was the fact that she was able to sleep through the night without having another bad dream. The last thing that she needed was for her nightmares to deprive her of sleep during a time when she was struggling to keep a normal sleep schedule was it was.
Since there wasn't anything pressing going on and her father noticed that she still didn't seem to be feeling much better by breakfast, he insisted that she stay home and could deal with any office matters from the house. That was the whole reason that he had given James after all so that she had someone to help her out when she couldn't come to the office.