Think you can skimp on an hour or two of shut-eye every night during the week and just pay it back on the weekends by sleeeping in? That’s not going to cut it if you want your brain to be at full functioning capacity, finds a new study published in the American Journal of Physiology- Endocrinology and Metabolism.

While other studies have found that a weekly sleep debt can be erased by sleeping for, say, 12 hours on a Friday night, this new research suggests that our brains may not fully recover even if we’re not feeling fatigued.

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The researchers in the new study subjected 30 volunteers in a sleep lab to six consecutive nights of six hours of sleep and then allowed them to sleep for 10 hours a night for three nights. The extra sleep helped lower elevated levels of stress hormones from the sleep deprivation and alleviated sleepiness.

But study participants had lower scores on cognitive tests—measuring alertness and reaction times—than they had at the beginning of the study when they were well rested.

Does this mean a sleep debt can never be paid back? Not at all, said study author Alexandros Vgontzas, director of the sleep research and treatment center at Penn State University. But it may take longer to settle up and restore the brain to full function than previously believed.

If you’re not getting enough sleep on most nights of the week, you can’t expect to repay the debt over a weekend. Your body needs more time to recalibrate its systems to adjust to the adequate sleep its now getting.

Hopefully, you’re starting the week on a restful note after getting plenty of extra sleep on the weekend. If you’re not, tack on an extra hour or two of sleep each night this week to help your body completely erase its debt.

After that, you’ll need to get ample sleep on most nights—preferably going to bed and waking at the same time each day without an alarm clock. That will ensure you won’t have fatigue, difficulty concentrating or other signs of sleep deprivation during the day.

If you don’t get enough sleep one night, taking a one to two hour nap before noon the next day can improve cognitive performance as well as reducing drowsiness and irritability.

Having a cup of caffeinated coffee can also improve your alertness, Vgontzas said, but use it in a pinch rather than as a substitute for adequate sleep.