Marginal gains is the theory, popularised by British Cycling performance director Sir Dave Brailsford, that improving every small element of performance by 1% produces dramatic cumulative results. Applied to British Cycling's programme from 2003, it helped produce five Tour de France victories in six years.
Improving by 1% every day for one year produces a result approximately 37 times better than the starting point (1.01^365 ≈ 37.8). Declining by 1% every day produces near-zero (0.99^365 ≈ 0.03). The same principle applies in reverse to small daily habits — either compounding forward or eroding backward.
Rather than seeking dramatic transformation, identify the smallest possible improvement to existing behaviours. Sleep 15 minutes earlier. Walk one stop further. Write one sentence more. Each is trivially achievable; over months, the cumulative effect is significant. James Clear's Atomic Habits builds directly on this framework.