The most common though that I see in BJJ is the cervical sprain or strain. The cervical spine is the most mobile segment of the spine however it puts it at a high risk of injury. Cervical sprains/strains are soft tissue injuries. A strain is linked to muscle, while a sprain is linked to tendons.
However, for some people, effects are much shorter and they have to take modafinil 2X per day (albeit this is not that common). For other people, effects can last 20-24 hours. It all depends on how your body processes it.
Modafinil comes in two strengths, 100 mg and 200 mg. It is usually taken in the morning to prevent daytime sleepiness or one hour before the start of a work shift. If you are using the medication to treat obstructive sleep apnea, you may also be given a continuous positive airway pressure (CPAP) machine.