Hi @Hans,
@Charliebrown gave some great information, and is absolutely correct.
You can utilize Auto mode, however that wouldn't always keep your airflow speed at a lower setting, as it would make adjustments as needed, to keep your air quality target in check while the machine is in use.
Setting your airflow speed manually is a good option; you can set the level from 1-10 and it'll stay there until you switch it or turn the machine off. Your air will be purified continuously, but the unit won't adjust to a drop in air quality. This is a great option if you want the noise or airflow from a fan while you sleep.
Night mode is also great for that use, as it not only dims the display, but the machine will run more quietly. If you set Auto mode, your unit may increase to airflow speed of 4, to adjust to air quality targets. However, you can enter the speed manually in night mode as well. You can customize the settings in your Dyson Link app.
Let us know if you have any further questions or concerns.
Best wishes,