Final answer:
During sleep, the fetus experiences a temporary decrease in variability.
Step-by-step explanation:
A temporary decrease in variability can occur when the fetus is in a sleep state. This is because during sleep, the fetus experiences a decrease in overall muscle tension and core body temperature, leading to a more synchronized brain wave activity. Stage 1 sleep, which is associated with relaxation and alpha waves, has less variability and is relatively easy to wake someone from. Therefore, the statement 'A temporary decrease in variability can occur when the fetus is in a sleep state' is true.