Is it bad to invest while the market is at such a high point?

I recently funded in full a roth IRA for 2017 (5,500) but I'm not sure if it is wise to invest it right now while the market is jumping to record highs. While I plan on investing in index funds such as the NASDAQ composite or S&P 500 composite and I know that the general rule is that time in the market is better than timing the market but I can't seem to shake the bad feeling about investing those funds at this point in time. Does anyone have some information about how to handle a situation like this?