Biopsychology: Biological Rhythms - Ultradian Rhythms

Ultradian rhythms last fewer than 24 hours and can be found in the pattern of human sleep. This cycle alternates between REM (rapid eye movement) and NREM (non-rapid movement) sleep and consists of five stages. The cycle starts at light sleep, progressing to deep sleep and then REM sleep, where brain waves speed up and dreaming occurs. This repeats itself about every 90 minutes throughout the night.

A complete sleep cycle goes through the four stages of NREM sleep before entering REM (Stage 5) and then repeating. Research using EEG has highlighted distinct brain waves patterns during the different stages of sleep.

• Stages 1 and 2 are ‘light sleep’ stages. During these stages brainwave patterns become slower and more rhythmic, starting with alpha waves progress to theta waves.
• Stages 3 and 4 are ‘deep sleep’ or slow wave sleep stages, where it is difficult to wake someone up. This stage is associated with slower delta waves.
• Finally, Stage 5 is REM (or dream) sleep. Here is the body is paralysed (to stop the person acting out their dream) and brain activity resembles that of an awake person.

On average, the entire cycle repeats every 90 minutes and a person can experience up to five full cycles in a night.

Evaluating Ultradian Rhythms

Individual Differences: The problem with studying sleep cycles is the differences observed in people, which make investigating patterns difficult. Tucker et al. (2007) found significant differences between participants in terms of the duration of each stage, particularly stages 3 and 4 (just before REM sleep). This demonstrates that there may be innate individual differences in ultradian rhythms, which means that it is worth focusing on these differences during investigations into sleep cycles.

In addition, this study was carried out in a controlled lab setting, which meant that the differences in the sleep patterns could not be attributed to situational factors, but only to biological differences between participants. While this study provide convincing support for the role of innate biological factors and ultradian rhythms, psychologists should examine other situational factors that may also play a role.

Additionally, the way in which such research is conducted may tell us little about ultradian rhythms in humans. When investigating sleep patterns, participants must be subjected to a specific level of control and be attached to monitors that measure such rhythms. This may be invasive for the participant, leading them to sleep in a way that does not represent their ordinary sleep cycle. This makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as their lack of ecological validity could lead to false conclusions being drawn.

An interesting case study indicates the flexibility of ultradian rhythms. Randy Gardener remained awake for 264 hours. While he experienced numerous problems such as blurred vision and disorganised speech, he coped rather well with the massive sleep loss. After this experience, Randy slept for just 15 hours and over several nights he recovered only 25% of his lost sleep. Interestingly, he recovered 70% of Stage 4 sleep, 50% of his REM sleep, and very little of the other stages. These results highlight the large degree of flexibility in terms of the different stages within the sleep cycle and the variable nature of this ultradian rhythm.