For doctors appointments I would specify doctors appointment. For anything else: insomnia (the original question), flu like symptoms, migraine... I would just label it not feeling well.
If the situation lasts more than a single day, or occurs more often than you are comfortable with, then see a doctor; and claim those hours for the appointment as a doctors appointment.
A longtime ago the company I worked for used the number of sick occurrences during the year into account when evaluating low level employees. The definition of sick occurrence required strict accounting of the reason for each absence:
- Not feeling well, so leaving early or arriving a few hours late: one occurrence
- Missing 4 hours for pre-surgical appointment, then 3 days for surgery and recovery, and two 3 hour post-op appointment 2 and 4 weeks later: one occurrence.
- going to physical therapy 2 times a week for 3 months: one occurrence.
If you missed 3 days in a row, you needed a note from a doctor.
They were looking to make sure some employees weren't treating the 10 days of sick a year as extra vacation, or that they were using it to leave early on some Fridays, or were masking frequent hangovers as being sick.
Now that most companies have combined short term illness and vacation into one pool, the paperwork requirements have virtually disappeared. My current company time card system only asks for you to apply your daily allotment of your 40 hours a week to charge numbers, holiday, and annual leave. If an employee were going to miss multiple days in a row they have a short term disability program and a long term disability program. These extra programs do require additional paperwork and forms from the doctors office.