[holmium]

It looks like there was a control group:

> However, an additional control was undertaken where colour contrast thresholds were measured in the morning and then re-measured 3 h later without exposure to 670 nm. This comprised of ten subjects (six Females and four males). To determine if there were significant shifts in colour contrast sensitivities across the day that were independent of 670 nm and might undermine outcome measures for their exposure, six subjects were repeatedly tested at 0, + 3, + 6 and + 9 h (four Females and two Males).

Figure 2 shows the (lack of) effect in the no-light control group at T0 and T+3hrs. Interestingly, they don't compare to baseline like in figure 1 with the AM light group. However, Figure 4 does show a lack of training effect when repeating the same test 4 times throughout the day within the same subject.

(Also, interesting that your first response to a paper in Nature is that they don't have a control group...)

Edit: And presumably they took the design of their study into account when calculating their statistics (https://www.stat.cmu.edu/~hseltman/309/Book/chapter14.pdf). Their methods seem to imply that, but I'm not familiar with it so I'll pray the reviewers checked for that lol.