Why Are Cockroaches More Commonly Seen at Night?

They Are Avoiding Light and Movement

Cockroaches are negatively phototactic — they move away from light — and they are thigmotactic, meaning they prefer to be in contact with surfaces on more than one side. Put those together and you get an insect that spends the day wedged in a crack in the dark and comes out when the kitchen is quiet.

It is not shyness. It is a survival strategy that works extremely well, and it is the reason an infestation is usually well established before anyone confirms it.

What a Daytime Sighting Means

This is the important part. A cockroach out in the open in daylight is generally a sign of pressure — the harbourage is crowded, or competition for it is pushing individuals out.

So one seen in daylight is a worse sign than several seen at night. If you have seen one on the kitchen floor at two in the afternoon, do not wait to see whether it happens again.

How to Check Without Waiting Up

Go into the kitchen at night, at least an hour after the light went off, and put the light on suddenly. Look at the floor near the kickboards, the wall behind the cooker and the gap beside the fridge. Anything moving fast for cover is what you are looking for.

Alternatively look for the evidence in daylight: droppings like coarse black pepper along cupboard edges, shed skins in cracks, and empty egg cases — small, brown, purse-shaped — wedged behind or under things.

Where They Are During the Day

Warm and tight. Behind and under the fridge, particularly around the motor. In the void behind kickboards. Around the boiler and its pipework. Inside the casing of a dishwasher or washing machine. In the hinge recesses of cupboard doors. Behind a wall-mounted microwave.

Almost all of those are within a couple of metres of food, water and heat, which is the combination the whole species is organised around.

Why This Changes the Treatment

If they are hiding by day, treating the surfaces you can see achieves very little. Gel bait goes into the harbourage and along the runs instead, and monitors are left so the population can be measured rather than guessed at.

It is also why "we have not seen one this week" is not evidence of anything. The monitors are.