One measure of market optimism is at tech-bubble highs. Here's why that's a worrying sign.

• By one measure, options markets are at record-levels of bullishness.
• A note from Sundial Capital's Jason Goepfert says that periods like this precede market downturn, Bloomberg previously reported.
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Options markets are so optimistic, they look like they did during the tech bubble of the early 2000s.

That's according to Sundial Capital's Jason Goepfert, who wrote in a Saturday note that the "level of leveraged speculation right now" only finds comparison in the dot-com bubble, Bloomberg previously reported.

Last week, buying of calls to open hit 24 million, the highest level ever, while investors sold to close 30% fewer calls than weeks prior, according to Goepfert's note, seen by Bloomberg.

Bought calls to open signify that traders have begun a new position; sold calls to close show that traders are exiting a position.

Bloomberg reported the difference between bought calls to open and sold calls to close is the largest it has ever been - a sign that contrarians may be dwindling amid strong bullish sentiment.

That difference is usually an omen of turbulence ahead for stocks, Goepfert's note said. The four times there has been a difference of 10 million between calls bought to open and calls sold to close, stocks typically fell a median of 4% within the following couple months.

To be sure, bearish calls predicting the demise of the longest-ever bull market are nothing new - few on Wall Street anticipated the S&P 500 index could run up 29% in 2019, Bloomberg reported.

But this optimism may too be exceptional. The options market has shown such a gap six times since mid-December, reported Bloomberg, citing research by Sundial's SentimenTrader using Options Clearing Corp. data.

"No other seven-week stretch has come even close to this level of speculative action," Goepfert's note read. "The only one that comes close ended in mid-February 2011, preceding a more than 6% loss in the S&P 500 over the next couple months, which only got worse in the months after that."