When you should buy a sleep accessory depends largely on what you need and why.
If this is your first time buying a mattress, then it's likely you'll also need to buy pillows, bedding, a bed frame, a duvet, and a mattress protector.
If you're replacing an old mattress, you probably have all of the basics already and can continue using them - unless you're upgrading or downgrading your bed to a different size.
Other sleep accessories can be bought when the need arises.
For example, if your mattress is too firm and you're struggling with back pain, but you can't return it, you could invest in a mattress topper to add some softness and comfort.
Or, if you and your partner have decided to sleep on two separate mattresses to form one double bed and are struggling with that annoying little gap in the middle, well, then a bed bridge is your answer.
As a general guideline, it's a good idea to invest in a sleep accessory if you've noticed that the quality of your sleep is decreasing.
There are dozens of accessories out there to help with all sorts of issues.
You must just find your lucky charm 💎