Look at the calendar first

The simplest way to tell them apart is timing.

SAD-related hypersomnia shows up in late winter (or fall/winter, depending on your hemisphere) and disappears when the seasons change. Idiopathic hypersomnia doesn’t care what month it is — it’s there year-round, same level of sleepiness whether it’s January or July.

Beyond that, check the company it keeps. SAD hypersomnia almost never travels alone. You’ll usually see the classic depression symptoms too: low mood, carb cravings, weight gain, low energy, social withdrawal. They all arrive together and leave together when spring hits. Idiopathic hypersomnia is more of a loner — maybe excessive sleep inertia (that “sleep drunkenness” where you can’t fully wake up) and long naps that don’t help.

If you’re still unsure, have the person track their sleepiness for a full year. If it’s consistently bad regardless of season, it’s likely idiopathic. If it magically clears up in May, it’s SAD.

Don’t overthink the diagnostic criteria — just pay attention to what time of year the problem actually exists.