If I find the urge to do something too great to be ignored, then it may be time to rebalance the portfolio. Suppose I have a certain percentage of our liquid assets in money market funds (maybe 10%), another chunk in bond assets of various maturities (maybe 30%), and yet another in equities (perhaps 60%, for instance). While money market account funds are stable, what tends to occur when the stock market is down is that people redeeming their stock holdings will put some of them into bonds or bond funds, thus lowering the yield on bonds and raising their market value. In addition, the loss of market value for the stocks or stock mutual funds portion of our net asset value reduces the whole portfolio, so even a stable asset, like a money market account, can rise as a fraction of the now diminished overall portfolio. In this scenario, then, the percentages will have gotten out of the intended balance, perhaps now standing at 12%, 34%, and 54%, respectively, for the money market, bond, and stock portions.
So, to restore the desired allocations, use some assets from money market funds and the rest from bond assets (which are up) to buy more equities (which are down) and thus get the nest egg back to 10/30/60% target levels for money market funds, bonds or bond mutual funds, and stocks or stock funds (the latter perhaps half invested with a low-cost S&P 500 Index exchange traded fund [ETF] and half in a low-cost small-cap ETF), in the process buying low (equities) and selling high (bonds).