One bad night of sleep does not automatically mean you should skip your run. But it can change how well your body is able to handle a hard session, so it is worth adjusting your expectations before you head out.
Start with how you actually feel, not just the number of hours you slept. You might sleep badly and still feel surprisingly normal. On another day, the same amount of sleep might leave you flat, irritable and struggling to concentrate.
If the run on your plan is easy, you can often still do it. Keep the effort genuinely easy and do not worry if your pace is slower than usual. The goal of the session is to get the easy aerobic work done without adding more stress than your body can handle.
Hard sessions need more thought. Intervals, tempo runs, hills and long runs all ask more of you physically and mentally. If you are clearly exhausted before you start, there is often more value in moving the session, shortening it or replacing it with an easy run.
That does not mean every poor night’s sleep should change your plan. Training would become impossible if you adjusted every session after one restless night. The important thing is to combine sleep with the other signals your body is giving you.
Ask yourself whether the bad sleep is a once-off or part of several difficult nights. Consider your stress levels, muscle soreness, motivation, resting heart rate if you track it, and how the first few minutes of the run feel. One signal on its own is not always meaningful. Several pointing in the same direction are harder to ignore.
If you decide to start the planned session, use the warm-up as a check-in. You do not have to make the final decision before you leave the house. If you warm up and feel normal, continue. If the effort feels unusually high or your legs feel dead, change the session.
This is particularly important during marathon training because fatigue builds over time. A bad night’s sleep on top of a demanding week can be very different from a bad night’s sleep after two easy days.
You are not losing fitness by changing one hard session. In many cases, you are protecting the quality of the work you will be able to do once you are recovered.
Training should respond to the body doing the training. The plan counts, but so does your readiness to handle what is written on it.