Day only
10/2026
Do the stock market's gains really all come overnight?
What did $1 in the S&P 500 grow to since 1993, held only at night? $25
A popular claim says that stocks rise while the exchange is closed and do not move while it is open. We split 33 years of the S&P 500, three other US funds and five markets abroad into the night and the day.
Do the stock market’s gains really all come overnight?
A stock market’s day has two halves. In this report, the night is the time from one day’s close to the next day’s open. The day is the time from that open to the close. In New York the day is six and a half hours of trading. The night is the other seventeen and a half hours, plus every weekend.
In 2011 Michael Kelly and Steven Clark studied the Nasdaq-100 fund from 1999 to 2006 (Kelly and Clark, 2011). They found that the fund paid much more than cash at night. In the day, it paid much less than cash.
The test in this report is simple. Each trading day, put $1 into a fund at the close and sell it at the next open. Do the same with a second $1 from each open to the close. The dollar that grows more shows which half of the day earned the return. One night’s return is small, so the comparison has meaning only over thousands of nights.
The claim is true for the returns and false as a trade. SPY is the oldest fund that follows the S&P 500. From February 1993, $1 held in SPY only at night grew to $25.35. A second $1 held only in the day grew to $1.27 (Fig. 1). In the Nasdaq-100 and Russell 2000 funds, the day dollar lost value. The same split occurs in Frankfurt, Paris, Tokyo, Hong Kong and Toronto.
To hold only the night, an investor must trade two times each day. On SPY, the night dollar ended below a dollar held all the time, which grew to $32.37. That result comes before any trading cost. The first fund that sold this trade lost 6.9% in just over a year, while the S&P 500 rose 22% (Greifeld, 2023). The fund then closed.
In each of four US stock funds, a dollar held only at night grew many times, and a dollar held only in the day grew much less or lost value.
Value of $1 held from each close to the next open (dot) and from each open to the close (ring), with dividends, from each fund's first full day to October 13, 2026: SPY from 1993, DIA from 1998, QQQ from 1999 and IWM from 2000.
Sources: daily prices via Yahoo Finance; ATOL calculation.
How we tested it
Each trading day has two returns. The night return is the change from the previous close to the open. The day return is the change from the open to the close. Together, the two returns give the usual close-to-close return. So they include every cent that an investor who held all the time made. A dividend goes into the night return, because a fund’s shares trade without the dividend from the open.
Figure 2 shows the split for SPY in the week of October 5, 2026. On Wednesday, October 7, SPY opened 0.43% below the Tuesday close. It then gained 0.19% by the close. So that night lost and that day gained. In the full week, the days earned more than the nights. One week is too short to show a pattern, but three decades are long enough.
Each trading day splits into a night, from the previous close to the open, and a day, from the open to the close.
SPY's opening and closing prices for the week of October 5, 2026, with the night's and the day's return in percent.
| Week of October 5, 2026 | Open, $ | Close, $ | Night, % | Day, % |
|---|---|---|---|---|
| Mon Oct 5 | 769.69 | 774.83 | 0.01 | 0.67 |
| Tue Oct 6 | 778.15 | 779.09 | 0.43 | 0.12 |
| Wed Oct 7 | 775.77 | 777.22 | -0.43 | 0.19 |
| Thu Oct 8 | 774.86 | 773.93 | -0.30 | -0.12 |
| Fri Oct 9 | 776.24 | 778.57 | 0.30 | 0.30 |
Sources: daily prices via Yahoo Finance; ATOL calculation.
The test uses four US exchange-traded funds. An investor with a brokerage account can buy each of them:
- SPY, which follows the S&P 500, from 1993;
- DIA, which follows the Dow Jones Industrial Average, from 1998;
- QQQ, which follows the Nasdaq-100, from 1999;
- IWM, which follows the Russell 2000, from 2000.
Each series ends at the close on October 13, 2026. The open of a fund is a real trade. So the night return of a fund is a price that an investor paid.
Outside the US, the test uses the main stock index of each market, because no single fund trades on all of these exchanges. The opening value of an index can be stale. The index provider calculates it before every stock in the index trades. Some data feeds only repeat the previous close. A year of an index goes into the test only if it meets two conditions:
- fewer than 5% of its opens are equal to the previous close;
- each half of the day has between 10% and 90% of the index’s daily variance, the measure of how much its returns move.
Five indices pass for 26 years or more. They are the DAX, the CAC 40, the Nikkei 225 from 2001, the Hang Seng and the S&P/TSX Composite. The FTSE 100, the S&P/ASX 200 and the Euro Stoxx 50 fail in most years, so the test does not use them.
All returns are compounded and given per year. A basis point is one hundredth of a percent. An average night on SPY earned 4.0 basis points.
Since 1993 almost all of the S&P 500’s return came at night
Over 33 years, SPY earned 10.1% a year at night and 0.7% a year in the day. On a log scale, the returns of the two halves add up exactly. On that scale, the night gave 93% of the fund’s return. An investor who held SPY only in the day from 1993 has little to show for it. An investor who held it only at night kept most of what the stock market paid.
Figure 3 shows that the split was steady. The night dollar rose in most of the 1990s. It fell in the bear markets of the early 2000s, 2008 and 2022, and it climbed again each time. The day dollar fell to 43 cents in February 2009. It then climbed back, and it is now worth $1.27.
In SPY, the night dollar did most of the growth, and the day dollar ended near where it began.
Value of $1 put into SPY on February 1, 1993 and held only at night, only in the day, or all the time, with dividends, at each month's end to October 13, 2026, on one shared scale.
Night only
All the time
Sources: daily prices via Yahoo Finance; ATOL calculation.
The split is larger in the two riskier funds. $1 held only in the day in QQQ from March 1999 is now worth 48 cents. The same dollar in IWM from May 2000 is worth 31 cents (Fig. 1). So both funds lost money in the day for more than 25 years, while their indices rose several times. DIA, the Dow fund, is the most stable of the four. It is the only fund in which the day earned more than 1% a year.
The result does not come from one or two good years. SPY’s night beat its day in 22 of the 32 full calendar years from 1994 (Fig. 4). The day lost money in 12 of those years. The night lost money in only 5.
SPY's night beat its day in 22 of 32 calendar years, by the widest margins in the late 1990s.
The night's return minus the day's return in each calendar year, in percentage points, 1994 to 2025; a bar below the line is a year in which the day earned more.
Sources: daily prices via Yahoo Finance; ATOL calculation.
The largest gaps came in the boom of the late 1990s. In 1999, SPY’s nights gained 45% and its days lost 17%. In 2000, the nights gained 27% and the days lost 29%. These two years made the largest part of the long-run gap. The next sections show how much of the pattern stays without them.
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