The short answer is that it depends on the ingredient, not on a blanket day-versus-night rule that applies to everything. Some actives have an ideal time. Others work perfectly well at any hour.
The ones that need a specific time
- Retinol: evening only, because of its sensitivity to sunlight
- Vitamin C: best during the day, as antioxidant protection against environmental stress
- Exfoliating acids: better at night, to reduce sun sensitivity
The ones that work at any time
- Niacinamide: suitable morning and evening
- Hyaluronic acid: works equally well at either time
- Calming peptides: generally flexible, though many people prefer the evening
The honest conclusion
You do not automatically need to buy a day serum and a night serum because marketing suggests it. You need to understand what each ingredient you already have actually does, and apply that logic. Some products genuinely require it. Others do not.
In the Lumyra system: 02 is the daytime step, 03 and 06 are the evening steps, and 04 goes both morning and night.