The aim of this study was to undertake a preliminary investigation of the novel hypothesis that weakened circadian rhythmicity is an aspect of the psychobiology of neuroticism (N). Two groups of subjects [young healthy females categorised as High N (N=8) or Low N (N=6)] were compared in terms of a classical circadian variable (the unmasked rhythm in core body temperature), and a validated psychological variable (diurnal rhythm in Positive Affect), under two experimental protocols (constant routine and ambulatory). As predicted, model-fitting analyses found that the High N group was characterised by attenuated circadian amplitudes in unmasked body temperature under both protocols. Also as hypothesised Positive Affect exhibited a circadian rhythm that was only significant in the Low N group. A stronger synchronisation between the biological and psychological rhythms was also observed in the Low N group. Results are discussed in terms of the implications of a possible circadian diathesis for disorders and complaints across the neurotic spectrum.