I'm taking methylin and I can't focus on my homework at night. Is it ok to take some more when I get home?

NO! Talk to your doctor before making any adjustments in medications!
To dream or not to d. Methylin is a stimulant as well as a drug which improves concentration. The later in the day you take methylin, or similar drugs, the longer it will take you to fall asleep. If you have developed tolerance to the insomnia effects of methylin, very likely this drug has lost some of its usefulness. A drug holiday of at least 5 days may restore your responsiveness to this medication.