Should I adjust my stock allocation when the market looks overvalued?

Generally no — trying to time your allocation around market valuations is far harder than it sounds and tends to hurt long-term returns. Markets can look "overvalued" for years while continuing to rise, and investors who pull back into cash often miss large gains and struggle to know when to get back in. Missing even a handful of the market's best days, which frequently cluster near downturns, can dramatically reduce decades of returns. A better approach is to set an allocation matched to your time horizon and risk capacity, then hold it through both bull and bear markets, rebalancing on your rule. Rebalancing already does a mild version of "buy low, sell high" automatically by trimming whatever's run up. If valuations genuinely worry you, the answer is a permanently more conservative allocation you can live with — not market-timing in and out.