It’s no secret that a strong research profile looks good on a residency application (see components of a strong residency application here).
So, when should you start?
In general, the earlier you start, the more you will produce. Research takes time, and there’s only so much you can cram into a small time slot.
On the flip side of that, research takes time and you should never sign up for a project if you don’t have the time to commit to it. Your academics are your primary focus in medical school. Having a strong research portfolio can only supplement academic excellence, it will not make up for it. The most important thing to do when joining a research lab is to set very clear expectations of what you can offer. How many hours can you honestly devote to the project while still excelling in your academics and taking care of your mental health? If you provide that information to your mentor, they should be able to help connect you with the right project(s).
Of course, that number will change over time. For example, I started research toward the end of my 1st year. When it was time to study for step 1, I made it clear to my research mentor that I was devoting my full energy to the exam and would not work on our projects during that time period. Third year I had very little time to spend on research, but did get some work done during the lighter rotations. During my research period, I focused all of my time on my research.
The amount of work I accomplished during my research period dwarfed everything I did during the first three years, but the first three years allowed me to make connections that were ultimately very beneficial for me.
So, do what you can with what you have, and when you have more - do more.
If you decide on taking a dedicated research period during medical school, here’s some advice on how to make the most of it.