The primary sign is a sleep–wake cycle longer than 24 hours, usually averaging a 25-26 hour cycle. In someone who is forcing their sleep with alarms, this may look like cyclical insomnia. When your Circadian Rhythm is not aligned with your schedule, you would have insomnia and symptoms of sleep deprivation, but when it is aligned, you would sleep fine.
In Sighted Non-24 patients, the retina may not react naturally to light. People with Sighted Non-24 can be subjectively sensitive to light (photophobia), or they can be under-sensitive to bright light.
Those of us with Sighted Non-24 typically sleep well and have an average habitual sleep duration of nine to ten hours, but we cannot sleep at the same time every day. This leads to a forward shift.