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When to take the USMLE step 1

So i'm almost definitely going to take the USMLE step 1. I'm just still deciding when to take it. I'm on a 6 year program with the 2nd half of 3rd year a dissertation.

I can take it at the end of my 3rd year summer (3 months long). That way I have 3 months of study time and part of the downtime of my dissertation to study. The negative on this is that I lose a full summer of possible research or clinical observership or traveling.

I can take it at the end of my 4th (1st clinical year). However, I only have 1 month of vacation that year and no dissertation downtime to study. The benefit is that I will have the full year of clinical experience that may help me with the exam. I've heard from people at my school that the 1st clinical year is really beneficial for the step 1? I've also read this same advice (i.e. to take it after 4th year from Digitalis on this forum). The downside of this is I lose a very important clinical year vacation (where I could possibly get a clinical elective in the US or Canada).

So this is my dilemma. My goal is to score as high as possible without sacrificing the activities i need to match to residency in Canada or the US. When is the recommended time to take this exam? Does the 1st clinical year really help that much?