Frequency analysis of circadian data

Hi everyone

i'm analysing the body core temperature, which should follow the circadian rythm. With a frequency analysis i want to find out the period of my data, to be able to fit a cosinor over my body core temperature curve. I have used adapted a code from a user in here, but i don't get the results i would expect (period should ± around 24h). The problem also is, that whenever i adapt my data ( at the moment i have data every minute), for example adapt it to data every 5 minutes, i get another result, which is odd because i would expect a similar result.

%% Read data
if isempty(path)==0
CoreTempPath=uigetdir('/Users/gabriela./Desktop/');
end

CoreTempData=CoreTemp_Open(CoreTempPath);
[l,j]=size(CoreTempData);

%% Applied Filters 
% hampel filter: median filter
for i=1:j;
[TempFilt,k]=hampel(CoreTempData(i).temp,50);
CoreTempData(i).k=k;
CoreTempData(i).tempFilt=TempFilt;
end
% moving average filter
a=1;
windowlength=25;
b=1/windowlength.*ones(1,windowlength);
for i=1:j;
TempFilt2=filtfilt(b,a,CoreTempData(i).tempFilt);
CoreTempData(i).tempFiltAverage=TempFilt2;
end

This are the first steps i do to read the data in. (there are for loops because i use to read several body core temperature data at once.

Then i average my data, so i get a data point every minute, with the following code

for i=1:j;
tt=timetable(CoreTempData(i).dateTime,CoreTempData(i).tempFiltAverage);
tt2=retime(tt,'minutely','mean') % mean every minute
CoreTempData(i).meantempwm=tt2.Var1; %mean Temperature over 1 minute with missing data
CoreTempData(i).meantime=tt2.Time; 
timetemp=fillmissing(tt2,'linear');%interpolating missing data
CoreTempData(i).meantemp1=timetemp.Var1;
end 

then i used the following code to do the frequency analysis and find the period

%% Frequency Analysis using fft --> power vs frequency

for i=1:j;
n = length(CoreTempData(i).meantemp1);
y = fft(CoreTempData(i).meantemp1);
y(1) = []; %sum of the data and can be removed

%code addapted. source:  https://ch.mathworks.com/help/matlab/examples/using-fft.html
n = length(y);
power = abs(y(1:floor(n/2))).^2; % power of first half of transform data
nyquist = 1/2;               % nyquist
freq = (1:n/2)/(n/2)*nyquist;    % equally spaced frequency grid
figure
plot(freq,power)
xlabel('Cycles/minute')
ylabel('Power')


figure
period = (1./(freq));
plot(period,power); 
xlabel('min/Cycle')
ylabel('Power')

hold on;
index=find(power==max(power));
mainPeriodStr=num2str(period(index));
plot(period(index),power(index),'r.', 'MarkerSize',25);
text(period(index)+2,power(index),['Period = ',mainPeriodStr,'min']);
hold off;
end

The result i get isn't quiet that what i would expect. But the thing is i don't know where the mistake in my code is. So if anyone could help, i would be very grateful!

Thank you