Evaluating the Statements

Let’s analyze the options regarding the effectiveness of light entrainment:

• A. Shorter exposures have a greater effect than longer exposures:
  Research shows that the greatest phase-resetting effects occur early in a light exposure, but longer exposures generally have a cumulative effect. Short, intense exposures can be effective, but this statement is not universally true for all scenarios.

• B. Bright light exposures have a greater effect than dim light:
  This is well established. Bright light is significantly more effective than dim light in entraining circadian rhythms in both humans and animals.

• C. Intermittent light exposures have a greater effect than consistent exposures:
  Studies show that intermittent bright light pulses can phase shift the circadian pacemaker as effectively, or even more efficiently per unit time, than continuous exposure. Intermittent exposures are thus highly effective.

• D. Dim light can affect entrainment relative to darkness:
  Dim light, while less effective than bright light, can still influence circadian entrainment when compared to complete darkness.

Which Combination Is Correct?

Based on current scientific understanding:

• Bright light (B) is more effective than dim light

• Intermittent light (C) can be as or more effective than continuous exposure

• Dim light (D) can affect entrainment compared to darkness

Therefore, the correct combination is:

(2) B and D only

• Bright light exposures have a greater effect than dim light.

• Dim light can affect entrainment relative to darkness.

This reflects the consensus that both the intensity and the presence of even low levels of light play important roles in circadian rhythm entrainment