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1954 - 195911990 - 19991

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Subject: search.filters.subject.Biological rhythms

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    Broad spectrum light and night-time mental performance : effects of intensity and duration : a thesis completed in partial fulfilment of the requirements for the degree of Master of Arts
    (Massey University, 1999) Baker, Tania
    The present study examined the effects of light intensity and duration on mental performance at night. A number of investigations have found light levels as low as 500 lux can have a significant impact on cognition, but there have been few, if any, systematic experiments that have investigated the potential trade-off between the intensity of the light and its duration. Light levels of 100 (normal room lighting), 300, 600 and 1,000 lux were paired with one of two different light exposure times: 15 and 60 minutes. Sixteen volunteers completed tests of critical thinking, simple maths, letter cancellation, recall, and recognition between 2300 and 0100 hours once a week for four consecutive weeks. Body temperature and subjective sleepiness levels were also recorded. The results showed that, in general, light intensities, irrespective of duration, of 300 and 600 lux had a positive effect on critical thinking and recognition memory. In contrast to some previous findings, there was little or no effect on sleepiness levels, core body temperature, recall, letter cancellation or the simple maths task. Surprisingly, the 1,000 lux light level had no effect on any of the tasks. It was concluded that changes in the intensity of broad-spectrum light can affect night-time cognitive performance, but that the intensity of the light cannot be traded for duration. However, further investigation of the manner in which light intensity is varied, either by distance from the light or by varying the brightness of the light source, is required before firm conclusions can be drawn.
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    A study of diurnal rhythms in milk and fat production in the cow : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science in the University of New Zealand
    (Massey University, 1954) Linklater, Peter Munro
    When cows are milked under commercial conditions milk yields are normally greater in the morning than in the evening, mainly because a longer time interval precedes the morning milking. In addition the percentage of fat in the milk is generally lower in the morning and higher in the evening. The alternation of high milk yields of low fat percentage, with lower milk yields of higher fat percentage, form a diurnal rhythm. This will depend partially on the length of time between morning and evening milkings, for the difference in yield and fat percentage is reduced as the time intervals approach equal length. There is left in the udder after a normal milking an appreciable quantity of milk of high fat percentage which has been termed "residual milk". In a hypothetical case if the volume and fat percentage of this milk remained constant after each milking, then the yield obtained from the udder would represent synthesis, and any diurnal rhythm would be caused solely by synthesis. However, neither the amount, nor the fat percentage of the residual milk remains constant at morning and evening milkings. Thus the diurnal rhythm may be attributed to two general causes, firstly synthesis, and superimposed on to this a diurnal variation in residual milk.