Apart from buying bitcoin during bull market, I think buying any other time can be profitable. The bitcoin market has trend, especially to the long time investors.It is always advisable to buy bitcoin when the price is low , so that when the price rises you will be able to make some profit.
When you buy your bitcoin when the market is already roaring ,it can make you loose money. Some people would continue buying bitcoin any time the price dips, and following market news will go a long way because you will always be informed when to buy, more especially when the market is in deep red.
You've got the core of a good investment approach: buying in fear and selling in greed. It's is impossible to time the absolute bottom and history has it that accumulating during bear markets and deep corrections ("deep red") rewards long-term holders. This dollar-cost averaging strategy softens out volatility and avoids the serious danger of buying at great top. Information is key, but discipline - buying when others are dreading - can truly build wealth in Bitcoin, using every market cycles advantageously.