Capture the pattern: evening only, or all day?
The most valuable detail in your question is the timing, so test it carefully. Note whether she hesitates only as light fades, or also on bright mornings and under full indoor lighting on the same stairs. Record how the pause looks: standing at the top measuring the first step, putting one paw forward and withdrawing it, sitting down and washing as if stalling, calling out, or retreating to wait until someone walks with her. Time roughly how long the hesitation lasts and what finally gets her moving — a light switched on, your presence on the stairs, or nothing changing except her own decision. If turning the hallway and stair lights on earlier removes the pause on several evenings running, write that down; it is one of the most useful experiments you can report.
Compare her confidence in bright and dim conditions elsewhere
Stairs are where the problem shows, but other low-light moments confirm or weaken the pattern. Watch whether she misjudges the edge of the bed or sofa at dusk, bumps a door frame she passes cleanly by day, moves more slowly through dim rooms, or startles when a hand approaches from one side in the evening. Note how she tracks a toy or treat in a lit room versus a shadowed one, and whether her pupils look unusually wide in ordinary indoor light. Also record stiffness after sleep and whether she bunches her back feet together step by step, since fading vision and sore joints can coexist.
Make the stairs safer tonight, not after the appointment
Do not wait for a diagnosis to reduce the risk of a fall. Leave a light on in the stairwell from late afternoon, fit a night light at the top and bottom if the route is still dim, and clear the steps of shoes, post, and laundry completely. Consider a gate or closed door at night if her essentials — litter box, water, food, and a warm bed — can all live on the floor where she spends the evening, so the stairs become optional rather than necessary. Check that stair carpet or treads are secure, because a loose edge that a young cat shrugged off can catch a cautious senior's paw mid-hesitation. If she insists on using them, walk slowly beside her rather than carrying her down, which can panic a cat who cannot see the drop.
Log the wider senior picture around the stairs
Keep brief parallel notes for the weeks before the check: appetite and how much she finishes, drinking, litter habits, weight as you feel it, sleep pattern, and her general confidence moving around the house. Record any other new behaviour — night calling, getting briefly stuck in a corner, missing a litter box she has used for years — because a cluster of small disorientations tells a different story from one stair problem. Also note when the hesitation began and whether it is lengthening, stable, or tied to late lighting. A short phone video of the pause can help too.
Ask targeted questions at the check, and call sooner on sudden change
At the appointment, lead with the light-versus-movement comparison: she manages at noon, hesitates at dusk, and walks confidently when the stair light is on — or whatever your log actually shows. Ask your vet to examine her eyes as well as her joints, spine, and muscle condition, and agree what to monitor next and which changes warrant a faster call. Contact the clinic sooner if the hesitation spreads suddenly to daylight, if she falls or slips on the stairs, stops eating, seems painful, or appears disoriented in familiar rooms, because a rapid shift in a senior cat is a different matter from a gradual evening pattern. Keep essentials on one floor until you have that advice if there is any doubt about her safety.
Filing the timed stair observations, light experiments, and wider notes in her KinsLe Pet Passport lets your vet read the pattern across evenings instead of reconstructing it from memory.