Calling in sick can be a valid reason to miss work in most places of employment. It would be helpful to know what the specific place of employment's policy is regarding both "sick days" and "personal days." Some places, for example, will allot a certain number of "personal days" to employees that can be used for many things so long as they don't go against company policy. In some cases, the "sick days" are included in the "personal days" so that an employee could call in sick and not have it count against them, so long as they still have personal days remaining.
Also, if an employee can provide proof of a doctor's diagnoses regarding illness, the absence will often be considered justified and accepted. Even if an absence is for a prolonged amount of time, a note from a doctor can sometimes be used to excuse an employee. In the case of a headache or migraine, if a person is diagnosed with migraines their absences may be considered more justified than someone who simply complains of headaches. However, once again this will greatly depend on the policy of the specific company that the employee works for.